path: root/lib/sqlalchemy/schema.py

# schema.py
# Copyright (C) 2005, 2006, 2007, 2008 Michael Bayer mike_mp@zzzcomputing.com
#
# This module is part of SQLAlchemy and is released under
# the MIT License: http://www.opensource.org/licenses/mit-license.php

"""The schema module provides the building blocks for database metadata.

Each element within this module describes a database entity which can be
created and dropped, or is otherwise part of such an entity.  Examples include
tables, columns, sequences, and indexes.

All entities are subclasses of :class:`~sqlalchemy.schema.SchemaItem`, and as defined
in this module they are intended to be agnostic of any vendor-specific
constructs.

A collection of entities are grouped into a unit called
:class:`~sqlalchemy.schema.MetaData`.  MetaData serves as a logical grouping of schema
elements, and can also be associated with an actual database connection such
that operations involving the contained elements can contact the database as
needed.

Two of the elements here also build upon their "syntactic" counterparts, which
are defined in :class:`~sqlalchemy.sql.expression.`, specifically
:class:`~sqlalchemy.schema.Table` and :class:`~sqlalchemy.schema.Column`.  Since these objects
are part of the SQL expression language, they are usable as components in SQL
expressions.

"""
import re, inspect
from sqlalchemy import types, exc, util, databases
from sqlalchemy.sql import expression, visitors

URL = None

__all__ = ['SchemaItem', 'Table', 'Column', 'ForeignKey', 'Sequence', 'Index',
           'ForeignKeyConstraint', 'PrimaryKeyConstraint', 'CheckConstraint',
           'UniqueConstraint', 'DefaultGenerator', 'Constraint', 'MetaData',
           'ThreadLocalMetaData', 'SchemaVisitor', 'PassiveDefault',
           'DefaultClause', 'FetchedValue', 'ColumnDefault', 'DDL']
__all__.sort()

class SchemaItem(visitors.Visitable):
    """Base class for items that define a database schema."""

    __visit_name__ = 'schema_item'
    quote = None

    def _init_items(self, *args):
        """Initialize the list of child items for this SchemaItem."""

        for item in args:
            if item is not None:
                item._set_parent(self)

    def _set_parent(self, parent):
        """Associate with this SchemaItem's parent object."""

        raise NotImplementedError()

    def get_children(self, **kwargs):
        """used to allow SchemaVisitor access"""
        return []

    def __repr__(self):
        return "%s()" % self.__class__.__name__

    @property
    def bind(self):
        """Return the connectable associated with this SchemaItem."""

        m = self.metadata
        return m and m.bind or None

    @property
    def info(self):
        try:
            return self._info
        except AttributeError:
            self._info = {}
            return self._info


def _get_table_key(name, schema):
    if schema is None:
        return name
    else:
        return schema + "." + name

class _TableSingleton(visitors.VisitableType):
    """A metaclass used by the ``Table`` object to provide singleton behavior."""

    def __call__(self, name, metadata, *args, **kwargs):
        schema = kwargs.get('schema', kwargs.get('owner', None))
        useexisting = kwargs.pop('useexisting', False)
        mustexist = kwargs.pop('mustexist', False)
        key = _get_table_key(name, schema)
        try:
            table = metadata.tables[key]
            if not useexisting and table._cant_override(*args, **kwargs):
                raise exc.InvalidRequestError(
                    "Table '%s' is already defined for this MetaData instance.  "
                    "Specify 'useexisting=True' to redefine options and "
                    "columns on an existing Table object." % key)
            else:
                table._init_existing(*args, **kwargs)
            return table
        except KeyError:
            if mustexist:
                raise exc.InvalidRequestError(
                    "Table '%s' not defined" % (key))
            try:
                return type.__call__(self, name, metadata, *args, **kwargs)
            except:
                if key in metadata.tables:
                    del metadata.tables[key]
                raise


class Table(SchemaItem, expression.TableClause):
    """Represent a table in a database."""

    __metaclass__ = _TableSingleton

    __visit_name__ = 'table'

    ddl_events = ('before-create', 'after-create', 'before-drop', 'after-drop')

    def __init__(self, name, metadata, *args, **kwargs):
        """
        Construct a Table.

        :param name: The name of this table as represented in the database. 

            This property, along with the *schema*, indicates the *singleton
            identity* of this table in relation to its parent :class:`MetaData`.
            Additional calls to :class:`Table` with the same name, metadata,
            and schema name will return the same :class:`Table` object.

            Names which contain no upper case characters
            will be treated as case insensitive names, and will not be quoted
            unless they are a reserved word.  Names with any number of upper
            case characters will be quoted and sent exactly.  Note that this
            behavior applies even for databases which standardize upper 
            case names as case insensitive such as Oracle.

        :param metadata: a :class:`MetaData` object which will contain this 
            table.  The metadata is used as a point of association of this table
            with other tables which are referenced via foreign key.  It also
            may be used to associate this table with a particular 
            :class:`~sqlalchemy.engine.base.Connectable`.

        :param \*args: Additional positional arguments are used primarily
            to add the list of :class:`Column` objects contained within this table.
            Similar to the style of a CREATE TABLE statement, other :class:`SchemaItem`
            constructs may be added here, including :class:`PrimaryKeyConstraint`,
            and :class:`ForeignKeyConstraint`.
            
        :param autoload: Defaults to False: the Columns for this table should be reflected
            from the database.  Usually there will be no Column objects in the
            constructor if this property is set.

        :param autoload_with: If autoload==True, this is an optional Engine or Connection
            instance to be used for the table reflection.  If ``None``, the
            underlying MetaData's bound connectable will be used.

        :param include_columns: A list of strings indicating a subset of columns to be loaded via
            the ``autoload`` operation; table columns who aren't present in
            this list will not be represented on the resulting ``Table``
            object.  Defaults to ``None`` which indicates all columns should
            be reflected.

        :param info: A dictionary which defaults to ``{}``.  A space to store application 
            specific data. This must be a dictionary.

        :param mustexist: When ``True``, indicates that this Table must already 
            be present in the given :class:`MetaData`` collection.

        :param prefixes:
            A list of strings to insert after CREATE in the CREATE TABLE
            statement.  They will be separated by spaces.

        :param quote: Force quoting of this table's name on or off, corresponding
            to ``True`` or ``False``.  When left at its default of ``None``,
            the column identifier will be quoted according to whether the name is
            case sensitive (identifiers with at least one upper case character are 
            treated as case sensitive), or if it's a reserved word.  This flag 
            is only needed to force quoting of a reserved word which is not known
            by the SQLAlchemy dialect.

        :param quote_schema: same as 'quote' but applies to the schema identifier.

        :param schema: The *schema name* for this table, which is required if the table
            resides in a schema other than the default selected schema for the
            engine's database connection.  Defaults to ``None``.

        :param useexisting: When ``True``, indicates that if this Table is already
            present in the given :class:`MetaData`, apply further arguments within
            the constructor to the existing :class:`Table`.  If this flag is not 
            set, an error is raised when the parameters of an existing :class:`Table`
            are overwritten.

        """
        super(Table, self).__init__(name)
        self.metadata = metadata
        self.schema = kwargs.pop('schema', kwargs.pop('owner', None))
        self.indexes = set()
        self.constraints = set()
        self._columns = expression.ColumnCollection()
        self.primary_key = PrimaryKeyConstraint()
        self._foreign_keys = util.OrderedSet()
        self.ddl_listeners = util.defaultdict(list)
        self.kwargs = {}
        if self.schema is not None:
            self.fullname = "%s.%s" % (self.schema, self.name)
        else:
            self.fullname = self.name

        autoload = kwargs.pop('autoload', False)
        autoload_with = kwargs.pop('autoload_with', None)
        include_columns = kwargs.pop('include_columns', None)

        self._set_parent(metadata)

        self.quote = kwargs.pop('quote', None)
        self.quote_schema = kwargs.pop('quote_schema', None)
        if kwargs.get('info'):
            self._info = kwargs.pop('info')

        self._prefixes = kwargs.pop('prefixes', [])

        self.__extra_kwargs(**kwargs)

        # load column definitions from the database if 'autoload' is defined
        # we do it after the table is in the singleton dictionary to support
        # circular foreign keys
        if autoload:
            if autoload_with:
                autoload_with.reflecttable(self, include_columns=include_columns)
            else:
                _bind_or_error(metadata).reflecttable(self, include_columns=include_columns)

        # initialize all the column, etc. objects.  done after reflection to
        # allow user-overrides
        self.__post_init(*args, **kwargs)

    def _init_existing(self, *args, **kwargs):
        autoload = kwargs.pop('autoload', False)
        autoload_with = kwargs.pop('autoload_with', None)
        schema = kwargs.pop('schema', None)
        if schema and schema != self.schema:
            raise exc.ArgumentError(
                "Can't change schema of existing table from '%s' to '%s'",
                (self.schema, schema))

        include_columns = kwargs.pop('include_columns', None)
        if include_columns:
            for c in self.c:
                if c.name not in include_columns:
                    self.c.remove(c)

        for key in ('quote', 'quote_schema'):
            if key in kwargs:
                setattr(self, key, kwargs.pop(key))

        if 'info' in kwargs:
            self._info = kwargs.pop('info')

        self.__extra_kwargs(**kwargs)
        self.__post_init(*args, **kwargs)

    def _cant_override(self, *args, **kwargs):
        """Return True if any argument is not supported as an override.

        Takes arguments that would be sent to Table.__init__, and returns
        True if any of them would be disallowed if sent to an existing
        Table singleton.
        """
        return bool(args) or bool(set(kwargs).difference(
            ['autoload', 'autoload_with', 'schema', 'owner']))

    def __extra_kwargs(self, **kwargs):
        # validate remaining kwargs that they all specify DB prefixes
        if len([k for k in kwargs
                if not re.match(r'^(?:%s)_' % '|'.join(databases.__all__), k)]):
            raise TypeError(
                "Invalid argument(s) for Table: %s" % repr(kwargs.keys()))
        self.kwargs.update(kwargs)

    def __post_init(self, *args, **kwargs):
        self._init_items(*args)

    @property
    def key(self):
        return _get_table_key(self.name, self.schema)

    def _set_primary_key(self, pk):
        if getattr(self, '_primary_key', None) in self.constraints:
            self.constraints.remove(self._primary_key)
        self._primary_key = pk
        self.constraints.add(pk)

    def primary_key(self):
        return self._primary_key
    primary_key = property(primary_key, _set_primary_key)

    def __repr__(self):
        return "Table(%s)" % ', '.join(
            [repr(self.name)] + [repr(self.metadata)] +
            [repr(x) for x in self.columns] +
            ["%s=%s" % (k, repr(getattr(self, k))) for k in ['schema']])

    def __str__(self):
        return _get_table_key(self.description, self.schema)

    def append_column(self, column):
        """Append a ``Column`` to this ``Table``."""

        column._set_parent(self)

    def append_constraint(self, constraint):
        """Append a ``Constraint`` to this ``Table``."""

        constraint._set_parent(self)

    def append_ddl_listener(self, event, listener):
        """Append a DDL event listener to this ``Table``.

        The ``listener`` callable will be triggered when this ``Table`` is
        created or dropped, either directly before or after the DDL is issued
        to the database.  The listener may modify the Table, but may not abort
        the event itself.

        Arguments are:

        event
          One of ``Table.ddl_events``; e.g. 'before-create', 'after-create',
          'before-drop' or 'after-drop'.

        listener
          A callable, invoked with three positional arguments:

          event
            The event currently being handled
          schema_item
            The ``Table`` object being created or dropped
          bind
            The ``Connection`` bueing used for DDL execution.

        Listeners are added to the Table's ``ddl_listeners`` attribute.
        """

        if event not in self.ddl_events:
            raise LookupError(event)
        self.ddl_listeners[event].append(listener)

    def _set_parent(self, metadata):
        metadata.tables[_get_table_key(self.name, self.schema)] = self
        self.metadata = metadata

    def get_children(self, column_collections=True, schema_visitor=False, **kwargs):
        if not schema_visitor:
            return expression.TableClause.get_children(
                self, column_collections=column_collections, **kwargs)
        else:
            if column_collections:
                return [c for c in self.columns]
            else:
                return []

    def exists(self, bind=None):
        """Return True if this table exists."""

        if bind is None:
            bind = _bind_or_error(self)

        def do(conn):
            return conn.dialect.has_table(conn, self.name, schema=self.schema)
        return bind.run_callable(do)

    def create(self, bind=None, checkfirst=False):
        """Issue a ``CREATE`` statement for this table.

        See also ``metadata.create_all()``.
        """
        self.metadata.create_all(bind=bind, checkfirst=checkfirst, tables=[self])

    def drop(self, bind=None, checkfirst=False):
        """Issue a ``DROP`` statement for this table.

        See also ``metadata.drop_all()``.
        """
        self.metadata.drop_all(bind=bind, checkfirst=checkfirst, tables=[self])

    def tometadata(self, metadata, schema=None):
        """Return a copy of this ``Table`` associated with a different ``MetaData``."""

        try:
            if not schema:
                schema = self.schema
            key = _get_table_key(self.name, schema)
            return metadata.tables[key]
        except KeyError:
            args = []
            for c in self.columns:
                args.append(c.copy(schema=schema))
            for c in self.constraints:
                args.append(c.copy(schema=schema))
            return Table(self.name, metadata, schema=schema, *args)

class Column(SchemaItem, expression.ColumnClause):
    """Represents a column in a database table."""

    __visit_name__ = 'column'

    def __init__(self, *args, **kwargs):
        """
        Construct a new ``Column`` object.
        
        :param name: The name of this column as represented in the database. 
          This argument may be the first positional argument, or specified
          via keyword.
          
          Names which contain no upper case characters
          will be treated as case insensitive names, and will not be quoted
          unless they are a reserved word.  Names with any number of upper
          case characters will be quoted and sent exactly.  Note that this
          behavior applies even for databases which standardize upper 
          case names as case insensitive such as Oracle.
          
          The name field may be omitted at construction time and applied
          later, at any time before the Column is associated with a 
          :class:`Table`.  This is to support convenient
          usage within the :mod:`~sqlalchemy.ext.declarative` extension.
          
        :param type\_: The column's type, indicated using an instance which 
          subclasses :class:`~sqlalchemy.types.AbstractType`.  If no arguments
          are required for the type, the class of the type can be sent
          as well, e.g.::
          
            # use a type with arguments
            Column('data', String(50))
            
            # use no arguments
            Column('level', Integer)
            
          The ``type`` argument may be the second positional argument
          or specified by keyword.

          If this column also contains a :class:`ForeignKey`,
          the type argument may be left as ``None`` in which case the
          type assigned will be that of the referenced column.

        :param \*args: Additional positional arguments include various 
          :class:`SchemaItem` derived constructs which will be applied 
          as options to the column.  These include instances of 
          :class:`Constraint`, :class:`ForeignKey`, :class:`ColumnDefault`, 
          and :class:`Sequence`.  In some cases an equivalent keyword 
          argument is available such as ``server_default``, ``default``
          and ``unique``.

        :param autoincrement: This flag may be set to ``False`` to disable
          SQLAlchemy indicating at the DDL level that an integer primary 
          key column should have autoincrementing behavior.  This 
          is an oft misunderstood flag and has no effect whatsoever unless
          all of the following conditions are met:
            
          * The column is of the :class:`~sqlalchemy.types.Integer` datatype.
          * The column has the ``primary_key`` flag set, or is otherwise
            a member of a :class:`PrimaryKeyConstraint` on this table.
          * a CREATE TABLE statement is being issued via :meth:`create()`
            or :meth:`create_all()`.  The flag has no relevance at any
            other time.
          * The database supports autoincrementing behavior, such as 
            Postgres or MySQL, and this behavior can be disabled (which does
            not include SQLite).

        :param default: A scalar, Python callable, or :class:`~sqlalchemy.sql.expression.ClauseElement`
          representing the *default value* for this column, which will be
          invoked upon insert if this column is otherwise not specified
          in the VALUES clause of the insert.  This is a shortcut
          to using :class:`ColumnDefault` as a positional argument.
          
          Contrast this argument to ``server_default`` which creates a 
          default generator on the database side.

        :param key: An optional string identifier which will identify this ``Column`` 
            object on the :class:`Table`.  When a key is provided, this is the
            only identifier referencing the ``Column`` within the application,
            including ORM attribute mapping; the ``name`` field is used only
            when rendering SQL.

        :param index: When ``True``, indicates that the column is indexed.
          This is a shortcut for using a :class:`Index` construct on the table.
          To specify indexes with explicit names or indexes that contain multiple 
          columns, use the :class:`Index` construct instead.

        :param info: A dictionary which defaults to ``{}``.  A space to store application 
          specific data. This must be a dictionary.

        :param nullable: If set to the default of ``True``, indicates the column
            will be rendered as allowing NULL, else it's rendered as NOT NULL.
            This parameter is only used when issuing CREATE TABLE statements.

        :param onupdate: A scalar, Python callable, or :class:`~sqlalchemy.sql.expression.ClauseElement`
            representing a default value to be applied to the column within UPDATE
            statements, which wil be invoked upon update if this column is not present
            in the SET clause of the update.  This is a shortcut to using 
            :class:`ColumnDefault` as a positional argument with ``for_update=True``.
            
        :param primary_key: If ``True``, marks this column as a primary key
            column.  Multiple columns can have this flag set to specify composite
            primary keys.  As an alternative, the primary key of a :class:`Table` can
            be specified via an explicit :class:`PrimaryKeyConstraint` object.

        :param server_default: A :class:`FetchedValue` instance, str, Unicode or
          :func:`~sqlalchemy.sql.expression.text` construct representing 
          the DDL DEFAULT value for the column.

          String types will be emitted as-is, surrounded by single quotes::

              Column('x', Text, server_default="val")

              x TEXT DEFAULT 'val'

          A :func:`~sqlalchemy.sql.expression.text` expression will be 
          rendered as-is, without quotes::

              Column('y', DateTime, server_default=text('NOW()'))0

              y DATETIME DEFAULT NOW()

          Strings and text() will be converted into a :class:`DefaultClause`
          object upon initialization.
          
          Use :class:`FetchedValue` to indicate that an already-existing column will generate
          a default value on the database side which will be available to SQLAlchemy 
          for post-fetch after inserts.  
          This construct does not specify any DDL and the implementation is 
          left to the database, such as via a trigger.

        :param server_onupdate:   A :class:`FetchedValue` instance representing 
            a database-side default generation function.  This indicates to 
            SQLAlchemy that a newly generated value will be available after updates.
            This construct does not specify any DDL and the implementation is 
            left to the database, such as via a trigger.

        :param quote: Force quoting of this column's name on or off, corresponding
           to ``True`` or ``False``.  When left at its default of ``None``,
           the column identifier will be quoted according to whether the name is
           case sensitive (identifiers with at least one upper case character are 
           treated as case sensitive), or if it's a reserved word.  This flag 
           is only needed to force quoting of a reserved word which is not known
           by the SQLAlchemy dialect.

        :param unique: When ``True``, indicates that this column contains a unique
            constraint, or if ``index`` is ``True`` as well, indicates that the
            :class:`Index` should be created with the unique flag.  To specify multiple
            columns in the constraint/index or to specify an explicit name,
            use the :class:`UniqueConstraint` or :class:`Index` constructs explicitly.

        """

        name = kwargs.pop('name', None)
        type_ = kwargs.pop('type_', None)
        if args:
            args = list(args)
            if isinstance(args[0], basestring):
                if name is not None:
                    raise exc.ArgumentError(
                        "May not pass name positionally and as a keyword.")
                name = args.pop(0)
        if args:
            coltype = args[0]
            
            # adjust for partials
            if util.callable(coltype):
                coltype = args[0]()

            if (isinstance(coltype, types.AbstractType) or
                (isinstance(coltype, type) and
                 issubclass(coltype, types.AbstractType))):
                if type_ is not None:
                    raise exc.ArgumentError(
                        "May not pass type_ positionally and as a keyword.")
                type_ = args.pop(0)

        super(Column, self).__init__(name, None, type_)
        self.args = args
        self.key = kwargs.pop('key', name)
        self.primary_key = kwargs.pop('primary_key', False)
        self.nullable = kwargs.pop('nullable', not self.primary_key)
        self.default = kwargs.pop('default', None)
        self.server_default = kwargs.pop('server_default', None)
        self.server_onupdate = kwargs.pop('server_onupdate', None)
        self.index = kwargs.pop('index', None)
        self.unique = kwargs.pop('unique', None)
        self.quote = kwargs.pop('quote', None)
        self.onupdate = kwargs.pop('onupdate', None)
        self.autoincrement = kwargs.pop('autoincrement', True)
        self.constraints = set()
        self.foreign_keys = util.OrderedSet()
        util.set_creation_order(self)

        if kwargs.get('info'):
            self._info = kwargs.pop('info')
        if kwargs:
            raise exc.ArgumentError(
                "Unknown arguments passed to Column: " + repr(kwargs.keys()))

    def __str__(self):
        if self.table is not None:
            if self.table.named_with_column:
                return (self.table.description + "." + self.description)
            else:
                return self.description
        else:
            return self.description

    def bind(self):
        return self.table.bind
    bind = property(bind)

    def references(self, column):
        """Return True if this Column references the given column via foreign key."""
        for fk in self.foreign_keys:
            if fk.references(column.table):
                return True
        else:
            return False

    def append_foreign_key(self, fk):
        fk._set_parent(self)

    def __repr__(self):
        kwarg = []
        if self.key != self.name:
            kwarg.append('key')
        if self.primary_key:
            kwarg.append('primary_key')
        if not self.nullable:
            kwarg.append('nullable')
        if self.onupdate:
            kwarg.append('onupdate')
        if self.default:
            kwarg.append('default')
        if self.server_default:
            kwarg.append('server_default')
        return "Column(%s)" % ', '.join(
            [repr(self.name)] + [repr(self.type)] +
            [repr(x) for x in self.foreign_keys if x is not None] +
            [repr(x) for x in self.constraints] +
            [(self.table and "table=<%s>" % self.table.description or "")] +
            ["%s=%s" % (k, repr(getattr(self, k))) for k in kwarg])

    def _set_parent(self, table):
        if self.name is None:
            raise exc.ArgumentError(
                "Column must be constructed with a name or assign .name "
                "before adding to a Table.")
        if self.key is None:
            self.key = self.name
        self.metadata = table.metadata
        if getattr(self, 'table', None) is not None:
            raise exc.ArgumentError("this Column already has a table!")

        self._pre_existing_column = table._columns.get(self.key)
        table._columns.replace(self)

        if self.primary_key:
            table.primary_key.replace(self)
        elif self.key in table.primary_key:
            raise exc.ArgumentError(
                "Trying to redefine primary-key column '%s' as a "
                "non-primary-key column on table '%s'" % (
                self.key, table.fullname))
            # if we think this should not raise an error, we'd instead do this:
            #table.primary_key.remove(self)
        self.table = table

        if self.index:
            if isinstance(self.index, basestring):
                raise exc.ArgumentError(
                    "The 'index' keyword argument on Column is boolean only. "
                    "To create indexes with a specific name, create an "
                    "explicit Index object external to the Table.")
            Index('ix_%s' % self._label, self, unique=self.unique)
        elif self.unique:
            if isinstance(self.unique, basestring):
                raise exc.ArgumentError(
                    "The 'unique' keyword argument on Column is boolean only. "
                    "To create unique constraints or indexes with a specific "
                    "name, append an explicit UniqueConstraint to the Table's "
                    "list of elements, or create an explicit Index object "
                    "external to the Table.")
            table.append_constraint(UniqueConstraint(self.key))

        toinit = list(self.args)
        if self.default is not None:
            if isinstance(self.default, ColumnDefault):
                toinit.append(self.default)
            else:
                toinit.append(ColumnDefault(self.default))
        if self.server_default is not None:
            if isinstance(self.server_default, FetchedValue):
                toinit.append(self.server_default)
            else:
                toinit.append(DefaultClause(self.server_default))
        if self.onupdate is not None:
            toinit.append(ColumnDefault(self.onupdate, for_update=True))
        if self.server_onupdate is not None:
            if isinstance(self.server_onupdate, FetchedValue):
                toinit.append(self.server_default)
            else:
                toinit.append(DefaultClause(self.server_onupdate,
                                            for_update=True))
        self._init_items(*toinit)
        self.args = None

    def copy(self, **kw):
        """Create a copy of this ``Column``, unitialized.

        This is used in ``Table.tometadata``.

        """
        return Column(self.name, self.type, self.default, key = self.key, primary_key = self.primary_key, nullable = self.nullable, quote=self.quote, index=self.index, autoincrement=self.autoincrement, *[c.copy(**kw) for c in self.constraints])

    def _make_proxy(self, selectable, name=None):
        """Create a *proxy* for this column.

        This is a copy of this ``Column`` referenced by a different parent
        (such as an alias or select statement).

        """
        fk = [ForeignKey(f._colspec) for f in self.foreign_keys]
        c = Column(name or self.name, self.type, self.default, key = name or self.key, primary_key = self.primary_key, nullable = self.nullable, quote=self.quote, *fk)
        c.table = selectable
        c.proxies = [self]
        c._pre_existing_column = self._pre_existing_column
        selectable.columns.add(c)
        if self.primary_key:
            selectable.primary_key.add(c)
        [c._init_items(f) for f in fk]
        return c

    def get_children(self, schema_visitor=False, **kwargs):
        if schema_visitor:
            return [x for x in (self.default, self.onupdate) if x is not None] + \
                list(self.foreign_keys) + list(self.constraints)
        else:
            return expression.ColumnClause.get_children(self, **kwargs)


class ForeignKey(SchemaItem):
    """Defines a column-level FOREIGN KEY constraint between two columns.

    ``ForeignKey`` is specified as an argument to a :class:`Column` object, e.g.::
    
        t = Table("remote_table", metadata, 
            Column("remote_id", ForeignKey("main_table.id"))
        )

    For a composite (multiple column) FOREIGN KEY, use a :class:`ForeignKeyConstraint`
    object specified at the level of the :class:`Table`.
    
    Further examples of foreign key configuration are in :ref:`metadata_foreignkeys`.

    """

    __visit_name__ = 'foreign_key'

    def __init__(self, column, constraint=None, use_alter=False, name=None, onupdate=None, ondelete=None, deferrable=None, initially=None, link_to_name=False):
        """
        Construct a column-level FOREIGN KEY.

        :param column: A single target column for the key relationship.  A :class:`Column`
          object or a column name as a string: ``tablename.columnkey`` or
          ``schema.tablename.columnkey``.  ``columnkey`` is the ``key`` which has been assigned
          to the column (defaults to the column name itself), unless ``link_to_name`` is ``True``
          in which case the rendered name of the column is used.

        :param constraint: Optional.  A parent :class:`ForeignKeyConstraint` object.  If not
          supplied, a :class:`ForeignKeyConstraint` will be automatically created
          and added to the parent table.

        :param name: Optional string.  An in-database name for the key if `constraint` is
          not provided.

        :param onupdate: Optional string.  If set, emit ON UPDATE <value> when issuing DDL
          for this constraint.  Typical values include CASCADE, DELETE and
          RESTRICT.

        :param ondelete: Optional string.  If set, emit ON DELETE <value> when issuing DDL
          for this constraint.  Typical values include CASCADE, DELETE and
          RESTRICT.

        :param deferrable: Optional bool.  If set, emit DEFERRABLE or NOT DEFERRABLE when
          issuing DDL for this constraint.

        :param initially: Optional string.  If set, emit INITIALLY <value> when issuing DDL
          for this constraint.
        
        :param link_to_name: if True, the string name given in ``column`` is the rendered
          name of the referenced column, not its locally assigned ``key``.
          
        :param use_alter: If True, do not emit this key as part of the CREATE TABLE
          definition.  Instead, use ALTER TABLE after table creation to add
          the key.  Useful for circular dependencies.
          
        """

        self._colspec = column
        self.constraint = constraint
        self.use_alter = use_alter
        self.name = name
        self.onupdate = onupdate
        self.ondelete = ondelete
        self.deferrable = deferrable
        self.initially = initially
        self.link_to_name = link_to_name

    def __repr__(self):
        return "ForeignKey(%r)" % self._get_colspec()

    def copy(self, schema=None):
        """Produce a copy of this ForeignKey object."""

        return ForeignKey(self._get_colspec(schema=schema))

    def _get_colspec(self, schema=None):
        if schema:
            return schema + "." + self.column.table.name + "." + self.column.key
        elif isinstance(self._colspec, basestring):
            return self._colspec
        else:
            return "%s.%s" % (self._colspec.table.fullname, self._colspec.key)
    target_fullname = property(_get_colspec)

    def references(self, table):
        """Return True if the given table is referenced by this ForeignKey."""
        return table.corresponding_column(self.column) is not None

    def get_referent(self, table):
        """Return the column in the given table referenced by this ForeignKey.

        Returns None if this ``ForeignKey`` does not reference the given table.

        """

        return table.corresponding_column(self.column)

    @util.memoized_property
    def column(self):
        # ForeignKey inits its remote column as late as possible, so tables
        # can be defined without dependencies
        if isinstance(self._colspec, basestring):
            # locate the parent table this foreign key is attached to.  we
            # use the "original" column which our parent column represents
            # (its a list of columns/other ColumnElements if the parent
            # table is a UNION)
            for c in self.parent.base_columns:
                if isinstance(c, Column):
                    parenttable = c.table
                    break
            else:
                raise exc.ArgumentError(
                    "Parent column '%s' does not descend from a "
                    "table-attached Column" % str(self.parent))
            m = re.match(r"^(.+?)(?:\.(.+?))?(?:\.(.+?))?$", self._colspec,
                         re.UNICODE)
            if m is None:
                raise exc.ArgumentError(
                    "Invalid foreign key column specification: %s" %
                    self._colspec)
            if m.group(3) is None:
                (tname, colname) = m.group(1, 2)
                schema = None
            else:
                (schema, tname, colname) = m.group(1, 2, 3)
            if _get_table_key(tname, schema) not in parenttable.metadata:
                raise exc.NoReferencedTableError(
                    "Could not find table '%s' with which to generate a "
                    "foreign key" % tname)
            table = Table(tname, parenttable.metadata,
                          mustexist=True, schema=schema)
                          
            _column = None
            if colname is None:
                # colname is None in the case that ForeignKey argument
                # was specified as table name only, in which case we
                # match the column name to the same column on the
                # parent.
                key = self.parent
                _column = table.c.get(self.parent.key, None)
            elif self.link_to_name:
                key = colname
                for c in table.c:
                    if c.name == colname:
                        _column = c
            else:
                key = colname
                _column = table.c.get(colname, None)

            if not _column:
                raise exc.NoReferencedColumnError(
                    "Could not create ForeignKey '%s' on table '%s': "
                    "table '%s' has no column named '%s'" % (
                    self._colspec, parenttable.name, table.name, key))

        elif hasattr(self._colspec, '__clause_element__'):
            _column = self._colspec.__clause_element__()
        else:
            _column = self._colspec

        # propagate TypeEngine to parent if it didn't have one
        if isinstance(self.parent.type, types.NullType):
            self.parent.type = _column.type
        return _column

    def _set_parent(self, column):
        if hasattr(self, 'parent'):
            raise exc.InvalidRequestError("This ForeignKey already has a parent !")
        self.parent = column

        if self.parent._pre_existing_column is not None:
            # remove existing FK which matches us
            for fk in self.parent._pre_existing_column.foreign_keys:
                if fk._colspec == self._colspec:
                    self.parent.table.foreign_keys.remove(fk)
                    self.parent.table.constraints.remove(fk.constraint)

        if self.constraint is None and isinstance(self.parent.table, Table):
            self.constraint = ForeignKeyConstraint(
                [], [], use_alter=self.use_alter, name=self.name,
                onupdate=self.onupdate, ondelete=self.ondelete,
                deferrable=self.deferrable, initially=self.initially)
            self.parent.table.append_constraint(self.constraint)
            self.constraint._append_fk(self)

        self.parent.foreign_keys.add(self)
        self.parent.table.foreign_keys.add(self)

class DefaultGenerator(SchemaItem):
    """Base class for column *default* values."""

    __visit_name__ = 'default_generator'

    def __init__(self, for_update=False, metadata=None):
        self.for_update = for_update
        self.metadata = util.assert_arg_type(metadata, (MetaData, type(None)), 'metadata')

    def _set_parent(self, column):
        self.column = column
        self.metadata = self.column.table.metadata
        if self.for_update:
            self.column.onupdate = self
        else:
            self.column.default = self

    def execute(self, bind=None, **kwargs):
        if bind is None:
            bind = _bind_or_error(self)
        return bind._execute_default(self, **kwargs)

    def __repr__(self):
        return "DefaultGenerator()"


class ColumnDefault(DefaultGenerator):
    """A plain default value on a column.

    This could correspond to a constant, a callable function, or a SQL clause.
    """

    def __init__(self, arg, **kwargs):
        super(ColumnDefault, self).__init__(**kwargs)
        if isinstance(arg, FetchedValue):
            raise exc.ArgumentError(
                "ColumnDefault may not be a server-side default type.")
        if util.callable(arg):
            arg = self._maybe_wrap_callable(arg)
        self.arg = arg

    def _maybe_wrap_callable(self, fn):
        """Backward compat: Wrap callables that don't accept a context."""

        if inspect.isfunction(fn):
            inspectable = fn
        elif inspect.isclass(fn):
            inspectable = fn.__init__
        elif hasattr(fn, '__call__'):
            inspectable = fn.__call__
        else:
            # probably not inspectable, try anyways.
            inspectable = fn
        try:
            argspec = inspect.getargspec(inspectable)
        except TypeError:
            return lambda ctx: fn()

        positionals = len(argspec[0])
        if inspect.ismethod(inspectable):
            positionals -= 1

        if positionals == 0:
            return lambda ctx: fn()

        defaulted = argspec[3] is not None and len(argspec[3]) or 0
        if positionals - defaulted > 1:
            raise exc.ArgumentError(
                "ColumnDefault Python function takes zero or one "
                "positional arguments")
        return fn

    def _visit_name(self):
        if self.for_update:
            return "column_onupdate"
        else:
            return "column_default"
    __visit_name__ = property(_visit_name)

    def __repr__(self):
        return "ColumnDefault(%s)" % repr(self.arg)

class Sequence(DefaultGenerator):
    """Represents a named database sequence."""

    __visit_name__ = 'sequence'

    def __init__(self, name, start=None, increment=None, schema=None,
                 optional=False, quote=None, **kwargs):
        super(Sequence, self).__init__(**kwargs)
        self.name = name
        self.start = start
        self.increment = increment
        self.optional = optional
        self.quote = quote
        self.schema = schema
        self.kwargs = kwargs

    def __repr__(self):
        return "Sequence(%s)" % ', '.join(
            [repr(self.name)] +
            ["%s=%s" % (k, repr(getattr(self, k)))
             for k in ['start', 'increment', 'optional']])

    def _set_parent(self, column):
        super(Sequence, self)._set_parent(column)
        column.sequence = self

    def create(self, bind=None, checkfirst=True):
        """Creates this sequence in the database."""

        if bind is None:
            bind = _bind_or_error(self)
        bind.create(self, checkfirst=checkfirst)

    def drop(self, bind=None, checkfirst=True):
        """Drops this sequence from the database."""

        if bind is None:
            bind = _bind_or_error(self)
        bind.drop(self, checkfirst=checkfirst)


class FetchedValue(object):
    """A default that takes effect on the database side."""

    def __init__(self, for_update=False):
        self.for_update = for_update

    def _set_parent(self, column):
        self.column = column
        if self.for_update:
            self.column.server_onupdate = self
        else:
            self.column.server_default = self

    def __repr__(self):
        return 'FetchedValue(for_update=%r)' % self.for_update


class DefaultClause(FetchedValue):
    """A DDL-specified DEFAULT column value."""

    def __init__(self, arg, for_update=False):
        util.assert_arg_type(arg, (basestring,
                                   expression.ClauseElement,
                                   expression._TextClause), 'arg')
        super(DefaultClause, self).__init__(for_update)
        self.arg = arg

    def __repr__(self):
        return "DefaultClause(%r, for_update=%r)" % (self.arg, self.for_update)

# alias; deprecated starting 0.5.0
PassiveDefault = DefaultClause


class Constraint(SchemaItem):
    """A table-level SQL constraint, such as a KEY.

    Implements a hybrid of dict/setlike behavior with regards to the list of
    underying columns.
    """

    __visit_name__ = 'constraint'

    def __init__(self, name=None, deferrable=None, initially=None):
        """Create a SQL constraint.

        name
          Optional, the in-database name of this ``Constraint``.

        deferrable
          Optional bool.  If set, emit DEFERRABLE or NOT DEFERRABLE when
          issuing DDL for this constraint.

        initially
          Optional string.  If set, emit INITIALLY <value> when issuing DDL
          for this constraint.
        """

        self.name = name
        self.columns = expression.ColumnCollection()
        self.deferrable = deferrable
        self.initially = initially

    def __contains__(self, x):
        return x in self.columns

    def contains_column(self, col):
        return self.columns.contains_column(col)

    def keys(self):
        return self.columns.keys()

    def __add__(self, other):
        return self.columns + other

    def __iter__(self):
        return iter(self.columns)

    def __len__(self):
        return len(self.columns)

    def copy(self, **kw):
        raise NotImplementedError()

class CheckConstraint(Constraint):
    """A table- or column-level CHECK constraint.

    Can be included in the definition of a Table or Column.
    """

    def __init__(self, sqltext, name=None, deferrable=None, initially=None):
        """Construct a CHECK constraint.

        sqltext
          A string containing the constraint definition.  Will be used
          verbatim.

        name
          Optional, the in-database name of the constraint.

        deferrable
          Optional bool.  If set, emit DEFERRABLE or NOT DEFERRABLE when
          issuing DDL for this constraint.

        initially
          Optional string.  If set, emit INITIALLY <value> when issuing DDL
          for this constraint.
        """

        super(CheckConstraint, self).__init__(name, deferrable, initially)
        if not isinstance(sqltext, basestring):
            raise exc.ArgumentError(
                "sqltext must be a string and will be used verbatim.")
        self.sqltext = sqltext

    def __visit_name__(self):
        if isinstance(self.parent, Table):
            return "check_constraint"
        else:
            return "column_check_constraint"
    __visit_name__ = property(__visit_name__)

    def _set_parent(self, parent):
        self.parent = parent
        parent.constraints.add(self)

    def copy(self, **kw):
        return CheckConstraint(self.sqltext, name=self.name)

class ForeignKeyConstraint(Constraint):
    """A table-level FOREIGN KEY constraint.

    Defines a single column or composite FOREIGN KEY ... REFERENCES
    constraint. For a no-frills, single column foreign key, adding a
    :class:`ForeignKey` to the definition of a :class:`Column` is a shorthand equivalent
    for an unnamed, single column :class:`ForeignKeyConstraint`.
    
    Examples of foreign key configuration are in :ref:`metadata_foreignkeys`.
    
    """
    __visit_name__ = 'foreign_key_constraint'

    def __init__(self, columns, refcolumns, name=None, onupdate=None, ondelete=None, use_alter=False, deferrable=None, initially=None, link_to_name=False):
        """Construct a composite-capable FOREIGN KEY.

        :param columns: A sequence of local column names.  The named columns must be defined
          and present in the parent Table.  The names should match the ``key`` given 
          to each column (defaults to the name) unless ``link_to_name`` is True.

        :param refcolumns: A sequence of foreign column names or Column objects.  The columns
          must all be located within the same Table.

        :param name: Optional, the in-database name of the key.

        :param onupdate: Optional string.  If set, emit ON UPDATE <value> when issuing DDL
          for this constraint.  Typical values include CASCADE, DELETE and
          RESTRICT.

        :param ondelete: Optional string.  If set, emit ON DELETE <value> when issuing DDL
          for this constraint.  Typical values include CASCADE, DELETE and
          RESTRICT.

        :param deferrable: Optional bool.  If set, emit DEFERRABLE or NOT DEFERRABLE when
          issuing DDL for this constraint.

        :param initially: Optional string.  If set, emit INITIALLY <value> when issuing DDL
          for this constraint.

        :param link_to_name: if True, the string name given in ``column`` is the rendered
          name of the referenced column, not its locally assigned ``key``.

        :param use_alter: If True, do not emit this key as part of the CREATE TABLE
          definition.  Instead, use ALTER TABLE after table creation to add
          the key.  Useful for circular dependencies.
          
        """
        super(ForeignKeyConstraint, self).__init__(name, deferrable, initially)
        self.__colnames = columns
        self.__refcolnames = refcolumns
        self.elements = util.OrderedSet()
        self.onupdate = onupdate
        self.ondelete = ondelete
        self.link_to_name = link_to_name
        if self.name is None and use_alter:
            raise exc.ArgumentError("Alterable ForeignKey/ForeignKeyConstraint requires a name")
        self.use_alter = use_alter

    def _set_parent(self, table):
        self.table = table
        if self not in table.constraints:
            table.constraints.add(self)
            for (c, r) in zip(self.__colnames, self.__refcolnames):
                self.append_element(c, r)

    def append_element(self, col, refcol):
        fk = ForeignKey(refcol, constraint=self, name=self.name, onupdate=self.onupdate, ondelete=self.ondelete, use_alter=self.use_alter, link_to_name=self.link_to_name)
        fk._set_parent(self.table.c[col])
        self._append_fk(fk)

    def _append_fk(self, fk):
        self.columns.add(self.table.c[fk.parent.key])
        self.elements.add(fk)

    def copy(self, **kw):
        return ForeignKeyConstraint([x.parent.name for x in self.elements], [x._get_colspec(**kw) for x in self.elements], name=self.name, onupdate=self.onupdate, ondelete=self.ondelete, use_alter=self.use_alter)

class PrimaryKeyConstraint(Constraint):
    """A table-level PRIMARY KEY constraint.

    Defines a single column or composite PRIMARY KEY constraint. For a
    no-frills primary key, adding ``primary_key=True`` to one or more
    ``Column`` definitions is a shorthand equivalent for an unnamed single- or
    multiple-column PrimaryKeyConstraint.
    """

    __visit_name__ = 'primary_key_constraint'

    def __init__(self, *columns, **kwargs):
        """Construct a composite-capable PRIMARY KEY.

        \*columns
          A sequence of column names.  All columns named must be defined and
          present within the parent Table.

        name
          Optional, the in-database name of the key.

        deferrable
          Optional bool.  If set, emit DEFERRABLE or NOT DEFERRABLE when
          issuing DDL for this constraint.

        initially
          Optional string.  If set, emit INITIALLY <value> when issuing DDL
          for this constraint.
        """

        constraint_args = dict(name=kwargs.pop('name', None),
                               deferrable=kwargs.pop('deferrable', None),
                               initially=kwargs.pop('initially', None))
        if kwargs:
            raise exc.ArgumentError(
                'Unknown PrimaryKeyConstraint argument(s): %s' %
                ', '.join(repr(x) for x in kwargs.keys()))

        super(PrimaryKeyConstraint, self).__init__(**constraint_args)
        self.__colnames = list(columns)

    def _set_parent(self, table):
        self.table = table
        table.primary_key = self
        for name in self.__colnames:
            self.add(table.c[name])

    def add(self, col):
        self.columns.add(col)
        col.primary_key = True
    append_column = add

    def replace(self, col):
        self.columns.replace(col)

    def remove(self, col):
        col.primary_key = False
        del self.columns[col.key]

    def copy(self, **kw):
        return PrimaryKeyConstraint(name=self.name, *[c.key for c in self])

    __hash__ = Constraint.__hash__
    
    def __eq__(self, other):
        return self.columns == other

class UniqueConstraint(Constraint):
    """A table-level UNIQUE constraint.

    Defines a single column or composite UNIQUE constraint. For a no-frills,
    single column constraint, adding ``unique=True`` to the ``Column``
    definition is a shorthand equivalent for an unnamed, single column
    UniqueConstraint.
    """

    __visit_name__ = 'unique_constraint'

    def __init__(self, *columns, **kwargs):
        """Construct a UNIQUE constraint.

        \*columns
          A sequence of column names.  All columns named must be defined and
          present within the parent Table.

        name
          Optional, the in-database name of the key.

        deferrable
          Optional bool.  If set, emit DEFERRABLE or NOT DEFERRABLE when
          issuing DDL for this constraint.

        initially
          Optional string.  If set, emit INITIALLY <value> when issuing DDL
          for this constraint.
        """

        constraint_args = dict(name=kwargs.pop('name', None),
                               deferrable=kwargs.pop('deferrable', None),
                               initially=kwargs.pop('initially', None))
        if kwargs:
            raise exc.ArgumentError(
                'Unknown UniqueConstraint argument(s): %s' %
                ', '.join(repr(x) for x in kwargs.keys()))

        super(UniqueConstraint, self).__init__(**constraint_args)
        self.__colnames = list(columns)

    def _set_parent(self, table):
        self.table = table
        table.constraints.add(self)
        for c in self.__colnames:
            self.append_column(table.c[c])

    def append_column(self, col):
        self.columns.add(col)

    def copy(self, **kw):
        return UniqueConstraint(name=self.name, *self.__colnames)

class Index(SchemaItem):
    """A table-level INDEX.

    Defines a composite (one or more column) INDEX. For a no-frills, single
    column index, adding ``index=True`` to the ``Column`` definition is
    a shorthand equivalent for an unnamed, single column Index.
    """

    __visit_name__ = 'index'

    def __init__(self, name, *columns, **kwargs):
        """Construct an index object.

        Arguments are:

        name
          The name of the index

        \*columns
          Columns to include in the index. All columns must belong to the same
          table, and no column may appear more than once.

        \**kwargs
          Keyword arguments include:

          unique
            Defaults to False: create a unique index.

          postgres_where
            Defaults to None: create a partial index when using PostgreSQL
        """

        self.name = name
        self.columns = []
        self.table = None
        self.unique = kwargs.pop('unique', False)
        self.kwargs = kwargs

        self._init_items(*columns)

    def _init_items(self, *args):
        for column in args:
            self.append_column(column)

    def _set_parent(self, table):
        self.table = table
        self.metadata = table.metadata
        table.indexes.add(self)

    def append_column(self, column):
        # make sure all columns are from the same table
        # and no column is repeated
        if self.table is None:
            self._set_parent(column.table)
        elif column.table != self.table:
            # all columns muse be from same table
            raise exc.ArgumentError(
                "All index columns must be from same table. "
                "%s is from %s not %s" % (column, column.table, self.table))
        elif column.name in [ c.name for c in self.columns ]:
            raise exc.ArgumentError(
                "A column may not appear twice in the "
                "same index (%s already has column %s)" % (self.name, column))
        self.columns.append(column)

    def create(self, bind=None):
        if bind is None:
            bind = _bind_or_error(self)
        bind.create(self)
        return self

    def drop(self, bind=None):
        if bind is None:
            bind = _bind_or_error(self)
        bind.drop(self)

    def __str__(self):
        return repr(self)

    def __repr__(self):
        return 'Index("%s", %s%s)' % (self.name,
                                      ', '.join(repr(c) for c in self.columns),
                                      (self.unique and ', unique=True') or '')

class MetaData(SchemaItem):
    """A collection of Tables and their associated schema constructs.

    Holds a collection of Tables and an optional binding to an ``Engine`` or
    ``Connection``.  If bound, the :class:`~sqlalchemy.schema.Table` objects
    in the collection and their columns may participate in implicit SQL
    execution.

    The `Table` objects themselves are stored in the `metadata.tables`
    dictionary.

    The ``bind`` property may be assigned to dynamically.  A common pattern is
    to start unbound and then bind later when an engine is available::

      metadata = MetaData()
      # define tables
      Table('mytable', metadata, ...)
      # connect to an engine later, perhaps after loading a URL from a
      # configuration file
      metadata.bind = an_engine

    MetaData is a thread-safe object after tables have been explicitly defined
    or loaded via reflection.

    .. index::
      single: thread safety; MetaData

    """

    __visit_name__ = 'metadata'

    ddl_events = ('before-create', 'after-create', 'before-drop', 'after-drop')

    def __init__(self, bind=None, reflect=False):
        """Create a new MetaData object.

        bind
          An Engine or Connection to bind to.  May also be a string or URL
          instance, these are passed to create_engine() and this MetaData will
          be bound to the resulting engine.

        reflect
          Optional, automatically load all tables from the bound database.
          Defaults to False. ``bind`` is required when this option is set.
          For finer control over loaded tables, use the ``reflect`` method of
          ``MetaData``.

        """
        self.tables = {}
        self.bind = bind
        self.metadata = self
        self.ddl_listeners = util.defaultdict(list)
        if reflect:
            if not bind:
                raise exc.ArgumentError(
                    "A bind must be supplied in conjunction with reflect=True")
            self.reflect()

    def __repr__(self):
        return 'MetaData(%r)' % self.bind

    def __contains__(self, key):
        return key in self.tables

    def __getstate__(self):
        return {'tables': self.tables}

    def __setstate__(self, state):
        self.tables = state['tables']
        self._bind = None

    def is_bound(self):
        """True if this MetaData is bound to an Engine or Connection."""

        return self._bind is not None

    @util.deprecated('Deprecated. Use ``metadata.bind = <engine>`` or '
                     '``metadata.bind = <url>``.')
    def connect(self, bind, **kwargs):
        """Bind this MetaData to an Engine.

        bind
          A string, ``URL``, ``Engine`` or ``Connection`` instance.  If a
          string or ``URL``, will be passed to ``create_engine()`` along with
          ``\**kwargs`` to produce the engine which to connect to.  Otherwise
          connects directly to the given ``Engine``.
          
        """
        global URL
        if URL is None:
            from sqlalchemy.engine.url import URL
        if isinstance(bind, (basestring, URL)):
            from sqlalchemy import create_engine
            self._bind = create_engine(bind, **kwargs)
        else:
            self._bind = bind

    def bind(self):
        """An Engine or Connection to which this MetaData is bound.

        This property may be assigned an ``Engine`` or ``Connection``, or
        assigned a string or URL to automatically create a basic ``Engine``
        for this bind with ``create_engine()``.
        
        """
        return self._bind

    def _bind_to(self, bind):
        """Bind this MetaData to an Engine, Connection, string or URL."""

        global URL
        if URL is None:
            from sqlalchemy.engine.url import URL

        if isinstance(bind, (basestring, URL)):
            from sqlalchemy import create_engine
            self._bind = create_engine(bind)
        else:
            self._bind = bind
    bind = property(bind, _bind_to)

    def clear(self):
        """Clear all Table objects from this MetaData."""
        # TODO: why have clear()/remove() but not all
        # other accesors/mutators for the tables dict ?
        self.tables.clear()

    def remove(self, table):
        """Remove the given Table object from this MetaData."""
        
        # TODO: scan all other tables and remove FK _column
        del self.tables[table.key]

    @util.deprecated('Deprecated. Use ``metadata.sorted_tables``')
    def table_iterator(self, reverse=True, tables=None):
        """Deprecated - use metadata.sorted_tables()."""
        
        from sqlalchemy.sql.util import sort_tables
        if tables is None:
            tables = self.tables.values()
        else:
            tables = set(tables).intersection(self.tables.values())
        ret = sort_tables(tables)
        if reverse:
            ret = reversed(ret)
        return iter(ret)
    
    @property
    def sorted_tables(self):
        """Returns a list of ``Table`` objects sorted in order of
        dependency.
        """
        from sqlalchemy.sql.util import sort_tables
        return sort_tables(self.tables.values())
        
    def reflect(self, bind=None, schema=None, only=None):
        """Load all available table definitions from the database.

        Automatically creates ``Table`` entries in this ``MetaData`` for any
        table available in the database but not yet present in the
        ``MetaData``.  May be called multiple times to pick up tables recently
        added to the database, however no special action is taken if a table
        in this ``MetaData`` no longer exists in the database.

        bind
          A :class:`~sqlalchemy.engine.base.Connectable` used to access the database; if None, uses the
          existing bind on this ``MetaData``, if any.

        schema
          Optional, query and reflect tables from an alterate schema.

        only
          Optional.  Load only a sub-set of available named tables.  May be
          specified as a sequence of names or a callable.

          If a sequence of names is provided, only those tables will be
          reflected.  An error is raised if a table is requested but not
          available.  Named tables already present in this ``MetaData`` are
          ignored.

          If a callable is provided, it will be used as a boolean predicate to
          filter the list of potential table names.  The callable is called
          with a table name and this ``MetaData`` instance as positional
          arguments and should return a true value for any table to reflect.

        """
        reflect_opts = {'autoload': True}
        if bind is None:
            bind = _bind_or_error(self)
            conn = None
        else:
            reflect_opts['autoload_with'] = bind
            conn = bind.contextual_connect()

        if schema is not None:
            reflect_opts['schema'] = schema

        available = util.OrderedSet(bind.engine.table_names(schema,
                                                            connection=conn))
        current = set(self.tables.keys())

        if only is None:
            load = [name for name in available if name not in current]
        elif util.callable(only):
            load = [name for name in available
                    if name not in current and only(name, self)]
        else:
            missing = [name for name in only if name not in available]
            if missing:
                s = schema and (" schema '%s'" % schema) or ''
                raise exc.InvalidRequestError(
                    'Could not reflect: requested table(s) not available '
                    'in %s%s: (%s)' % (bind.engine.url, s, ', '.join(missing)))
            load = [name for name in only if name not in current]

        for name in load:
            Table(name, self, **reflect_opts)

    def append_ddl_listener(self, event, listener):
        """Append a DDL event listener to this ``MetaData``.

        The ``listener`` callable will be triggered when this ``MetaData`` is
        involved in DDL creates or drops, and will be invoked either before
        all Table-related actions or after.

        Arguments are:

        event
          One of ``MetaData.ddl_events``; 'before-create', 'after-create',
          'before-drop' or 'after-drop'.
        listener
          A callable, invoked with three positional arguments:

          event
            The event currently being handled
          schema_item
            The ``MetaData`` object being operated upon
          bind
            The ``Connection`` bueing used for DDL execution.

        Listeners are added to the MetaData's ``ddl_listeners`` attribute.

        Note: MetaData listeners are invoked even when ``Tables`` are created
        in isolation.  This may change in a future release. I.e.::

          # triggers all MetaData and Table listeners:
          metadata.create_all()

          # triggers MetaData listeners too:
          some.table.create()

        """
        if event not in self.ddl_events:
            raise LookupError(event)
        self.ddl_listeners[event].append(listener)

    def create_all(self, bind=None, tables=None, checkfirst=True):
        """Create all tables stored in this metadata.

        Conditional by default, will not attempt to recreate tables already
        present in the target database.

        bind
          A :class:`~sqlalchemy.engine.base.Connectable` used to access the database; if None, uses the
          existing bind on this ``MetaData``, if any.

        tables
          Optional list of ``Table`` objects, which is a subset of the total
          tables in the ``MetaData`` (others are ignored).

        checkfirst
          Defaults to True, don't issue CREATEs for tables already present
          in the target database.
          
        """
        if bind is None:
            bind = _bind_or_error(self)
        for listener in self.ddl_listeners['before-create']:
            listener('before-create', self, bind)
        bind.create(self, checkfirst=checkfirst, tables=tables)
        for listener in self.ddl_listeners['after-create']:
            listener('after-create', self, bind)

    def drop_all(self, bind=None, tables=None, checkfirst=True):
        """Drop all tables stored in this metadata.

        Conditional by default, will not attempt to drop tables not present in
        the target database.

        bind
          A :class:`~sqlalchemy.engine.base.Connectable` used to access the database; if None, uses
          the existing bind on this ``MetaData``, if any.

        tables
          Optional list of ``Table`` objects, which is a subset of the
          total tables in the ``MetaData`` (others are ignored).

        checkfirst
          Defaults to True, only issue DROPs for tables confirmed to be present
          in the target database.

        """
        if bind is None:
            bind = _bind_or_error(self)
        for listener in self.ddl_listeners['before-drop']:
            listener('before-drop', self, bind)
        bind.drop(self, checkfirst=checkfirst, tables=tables)
        for listener in self.ddl_listeners['after-drop']:
            listener('after-drop', self, bind)

class ThreadLocalMetaData(MetaData):
    """A MetaData variant that presents a different ``bind`` in every thread.

    Makes the ``bind`` property of the MetaData a thread-local value, allowing
    this collection of tables to be bound to different ``Engine``
    implementations or connections in each thread.

    The ThreadLocalMetaData starts off bound to None in each thread.  Binds
    must be made explicitly by assigning to the ``bind`` property or using
    ``connect()``.  You can also re-bind dynamically multiple times per
    thread, just like a regular ``MetaData``.

    """

    __visit_name__ = 'metadata'

    def __init__(self):
        """Construct a ThreadLocalMetaData."""

        self.context = util.ThreadLocal()
        self.__engines = {}
        super(ThreadLocalMetaData, self).__init__()

    @util.deprecated('Deprecated. Use ``metadata.bind = <engine>`` or '
                     '``metadata.bind = <url>``.')
    def connect(self, bind, **kwargs):
        """Bind to an Engine in the caller's thread.

        bind
          A string, ``URL``, ``Engine`` or ``Connection`` instance.  If a
          string or ``URL``, will be passed to ``create_engine()`` along with
          ``\**kwargs`` to produce the engine which to connect to.  Otherwise
          connects directly to the given ``Engine``.
        """

        global URL
        if URL is None:
            from sqlalchemy.engine.url import URL

        if isinstance(bind, (basestring, URL)):
            try:
                engine = self.__engines[bind]
            except KeyError:
                from sqlalchemy import create_engine
                engine = create_engine(bind, **kwargs)
            bind = engine
        self._bind_to(bind)

    def bind(self):
        """The bound Engine or Connection for this thread.

        This property may be assigned an Engine or Connection, or assigned a
        string or URL to automatically create a basic Engine for this bind
        with ``create_engine()``."""

        return getattr(self.context, '_engine', None)

    def _bind_to(self, bind):
        """Bind to a Connectable in the caller's thread."""

        global URL
        if URL is None:
            from sqlalchemy.engine.url import URL

        if isinstance(bind, (basestring, URL)):
            try:
                self.context._engine = self.__engines[bind]
            except KeyError:
                from sqlalchemy import create_engine
                e = create_engine(bind)
                self.__engines[bind] = e
                self.context._engine = e
        else:
            # TODO: this is squirrely.  we shouldnt have to hold onto engines
            # in a case like this
            if bind not in self.__engines:
                self.__engines[bind] = bind
            self.context._engine = bind

    bind = property(bind, _bind_to)

    def is_bound(self):
        """True if there is a bind for this thread."""
        return (hasattr(self.context, '_engine') and
                self.context._engine is not None)

    def dispose(self):
        """Dispose all bound engines, in all thread contexts."""

        for e in self.__engines.values():
            if hasattr(e, 'dispose'):
                e.dispose()

class SchemaVisitor(visitors.ClauseVisitor):
    """Define the visiting for ``SchemaItem`` objects."""

    __traverse_options__ = {'schema_visitor':True}


class DDL(object):
    """A literal DDL statement.

    Specifies literal SQL DDL to be executed by the database.  DDL objects can
    be attached to ``Tables`` or ``MetaData`` instances, conditionally
    executing SQL as part of the DDL lifecycle of those schema items.  Basic
    templating support allows a single DDL instance to handle repetitive tasks
    for multiple tables.

    Examples::

      tbl = Table('users', metadata, Column('uid', Integer)) # ...
      DDL('DROP TRIGGER users_trigger').execute_at('before-create', tbl)

      spow = DDL('ALTER TABLE %(table)s SET secretpowers TRUE', on='somedb')
      spow.execute_at('after-create', tbl)

      drop_spow = DDL('ALTER TABLE users SET secretpowers FALSE')
      connection.execute(drop_spow)
    """

    def __init__(self, statement, on=None, context=None, bind=None):
        """Create a DDL statement.

        statement
          A string or unicode string to be executed.  Statements will be
          processed with Python's string formatting operator.  See the
          ``context`` argument and the ``execute_at`` method.

          A literal '%' in a statement must be escaped as '%%'.

          SQL bind parameters are not available in DDL statements.

        on
          Optional filtering criteria.  May be a string or a callable
          predicate.  If a string, it will be compared to the name of the
          executing database dialect::

            DDL('something', on='postgres')

          If a callable, it will be invoked with three positional arguments:

            event
              The name of the event that has triggered this DDL, such as
              'after-create' Will be None if the DDL is executed explicitly.

            schema_item
              A SchemaItem instance, such as ``Table`` or ``MetaData``. May be
              None if the DDL is executed explicitly.

            connection
              The ``Connection`` being used for DDL execution

          If the callable returns a true value, the DDL statement will be
          executed.

        context
          Optional dictionary, defaults to None.  These values will be
          available for use in string substitutions on the DDL statement.

        bind
          Optional. A :class:`~sqlalchemy.engine.base.Connectable`, used by default when ``execute()``
          is invoked without a bind argument.
          
        """

        if not isinstance(statement, basestring):
            raise exc.ArgumentError(
                "Expected a string or unicode SQL statement, got '%r'" %
                statement)
        if (on is not None and
            (not isinstance(on, basestring) and not util.callable(on))):
            raise exc.ArgumentError(
                "Expected the name of a database dialect or a callable for "
                "'on' criteria, got type '%s'." % type(on).__name__)

        self.statement = statement
        self.on = on
        self.context = context or {}
        self._bind = bind

    def execute(self, bind=None, schema_item=None):
        """Execute this DDL immediately.

        Executes the DDL statement in isolation using the supplied
        :class:`~sqlalchemy.engine.base.Connectable` or :class:`~sqlalchemy.engine.base.Connectable` assigned to the ``.bind`` property,
        if not supplied.  If the DDL has a conditional ``on`` criteria, it
        will be invoked with None as the event.

        bind
          Optional, an ``Engine`` or ``Connection``.  If not supplied, a
          valid :class:`~sqlalchemy.engine.base.Connectable` must be present in the ``.bind`` property.

        schema_item
          Optional, defaults to None.  Will be passed to the ``on`` callable
          criteria, if any, and may provide string expansion data for the
          statement. See ``execute_at`` for more information.
        """

        if bind is None:
            bind = _bind_or_error(self)
        # no SQL bind params are supported
        if self._should_execute(None, schema_item, bind):
            executable = expression.text(self._expand(schema_item, bind))
            return bind.execute(executable)
        else:
            bind.engine.logger.info("DDL execution skipped, criteria not met.")

    def execute_at(self, event, schema_item):
        """Link execution of this DDL to the DDL lifecycle of a SchemaItem.

        Links this ``DDL`` to a ``Table`` or ``MetaData`` instance, executing
        it when that schema item is created or dropped.  The DDL statement
        will be executed using the same Connection and transactional context
        as the Table create/drop itself.  The ``.bind`` property of this
        statement is ignored.

        event
          One of the events defined in the schema item's ``.ddl_events``;
          e.g. 'before-create', 'after-create', 'before-drop' or 'after-drop'

        schema_item
          A Table or MetaData instance

        When operating on Table events, the following additional ``statement``
        string substitions are available::

            %(table)s  - the Table name, with any required quoting applied
            %(schema)s - the schema name, with any required quoting applied
            %(fullname)s - the Table name including schema, quoted if needed

        The DDL's ``context``, if any, will be combined with the standard
        substutions noted above.  Keys present in the context will override
        the standard substitutions.

        A DDL instance can be linked to any number of schema items. The
        statement subsitution support allows for DDL instances to be used in a
        template fashion.

        ``execute_at`` builds on the ``append_ddl_listener`` interface of
        MetaDta and Table objects.

        Caveat: Creating or dropping a Table in isolation will also trigger
        any DDL set to ``execute_at`` that Table's MetaData.  This may change
        in a future release.
        """

        if not hasattr(schema_item, 'ddl_listeners'):
            raise exc.ArgumentError(
                "%s does not support DDL events" % type(schema_item).__name__)
        if event not in schema_item.ddl_events:
            raise exc.ArgumentError(
                "Unknown event, expected one of (%s), got '%r'" %
                (', '.join(schema_item.ddl_events), event))
        schema_item.ddl_listeners[event].append(self)
        return self

    def bind(self):
        """An Engine or Connection to which this DDL is bound.

        This property may be assigned an ``Engine`` or ``Connection``, or
        assigned a string or URL to automatically create a basic ``Engine``
        for this bind with ``create_engine()``.
        """
        return self._bind

    def _bind_to(self, bind):
        """Bind this MetaData to an Engine, Connection, string or URL."""

        global URL
        if URL is None:
            from sqlalchemy.engine.url import URL

        if isinstance(bind, (basestring, URL)):
            from sqlalchemy import create_engine
            self._bind = create_engine(bind)
        else:
            self._bind = bind
    bind = property(bind, _bind_to)

    def __call__(self, event, schema_item, bind):
        """Execute the DDL as a ddl_listener."""

        if self._should_execute(event, schema_item, bind):
            statement = expression.text(self._expand(schema_item, bind))
            return bind.execute(statement)

    def _expand(self, schema_item, bind):
        return self.statement % self._prepare_context(schema_item, bind)

    def _should_execute(self, event, schema_item, bind):
        if self.on is None:
            return True
        elif isinstance(self.on, basestring):
            return self.on == bind.engine.name
        else:
            return self.on(event, schema_item, bind)

    def _prepare_context(self, schema_item, bind):
        # table events can substitute table and schema name
        if isinstance(schema_item, Table):
            context = self.context.copy()

            preparer = bind.dialect.identifier_preparer
            path = preparer.format_table_seq(schema_item)
            if len(path) == 1:
                table, schema = path[0], ''
            else:
                table, schema = path[-1], path[0]

            context.setdefault('table', table)
            context.setdefault('schema', schema)
            context.setdefault('fullname', preparer.format_table(schema_item))
            return context
        else:
            return self.context

    def __repr__(self):
        return '<%s@%s; %s>' % (
            type(self).__name__, id(self),
            ', '.join([repr(self.statement)] +
                      ['%s=%r' % (key, getattr(self, key))
                       for key in ('on', 'context')
                       if getattr(self, key)]))


def _bind_or_error(schemaitem):
    bind = schemaitem.bind
    if not bind:
        name = schemaitem.__class__.__name__
        label = getattr(schemaitem, 'fullname',
                        getattr(schemaitem, 'name', None))
        if label:
            item = '%s %r' % (name, label)
        else:
            item = name
        if isinstance(schemaitem, (MetaData, DDL)):
            bindable = "the %s's .bind" % name
        else:
            bindable = "this %s's .metadata.bind" % name

        msg = ('The %s is not bound to an Engine or Connection.  '
               'Execution can not proceed without a database to execute '
               'against.  Either execute with an explicit connection or '
               'assign %s to enable implicit execution.') % (item, bindable)
        raise exc.UnboundExecutionError(msg)
    return bind