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 [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
[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 [sqlalchemy.sql.expression#], specifically
[sqlalchemy.schema#Table] and [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, exceptions, 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',
           'ColumnDefault', 'DDL']

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

    __metaclass__ = expression._FigureVisitName

    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__

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

        m = self.metadata
        return m and m.bind or None
    bind = property(bind)

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


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

class _TableSingleton(expression._FigureVisitName):
    """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 exceptions.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 exceptions.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 relational database table."""

    __metaclass__ = _TableSingleton

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

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

        Table objects can be constructed directly.  Arguments are:

        name
          The name of this table, exactly as it appears, or will appear, in
          the database.

          This property, along with the *schema*, indicates the *singleton
          identity* of this table.

          Further tables constructed with the same name/schema combination
          will return the same Table instance.

        \*args
          Should contain a listing of the Column objects for this table.

        \**kwargs
          kwargs include:

          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``.

          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.

          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.

          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.

          info
            Defaults to {}: A space to store application specific data; this
            must be a dictionary.

          mustexist
            Defaults to False: indicates that this Table must already have
            been defined elsewhere in the application, else an exception is
            raised.

          useexisting
            Defaults to False: indicates that if this Table was already
            defined elsewhere in the application, disregard the rest of the
            constructor arguments.

          owner
            Deprecated; this is an oracle-only argument - "schema" should
            be used in its place.

          quote
            When True, indicates that the Table identifier must be quoted.
            This flag does *not* disable quoting; for case-insensitive names,
            use an all lower case identifier.

          quote_schema
            When True, indicates that the schema identifier must be quoted.
            This flag does *not* disable quoting; for case-insensitive names,
            use an all lower case identifier.
        """

        super(Table, self).__init__(name)
        self.metadata = metadata
        self.schema = kwargs.pop('schema', kwargs.pop('owner', None))
        self.indexes = util.Set()
        self.constraints = util.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.__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 exceptions.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)

        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(util.Set(kwargs).difference(
            ['autoload', 'autoload_with', 'schema', 'owner']))

    def __extra_kwargs(self, **kwargs):
        self.quote = kwargs.pop('quote', False)
        self.quote_schema = kwargs.pop('quote_schema', False)
        if kwargs.get('info'):
            self._info = kwargs.pop('info')

        # 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)

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

    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 schema is None:
                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())
            for c in self.constraints:
                args.append(c.copy())
            return Table(self.name, metadata, schema=schema, *args)

class Column(SchemaItem, expression._ColumnClause):
    """Represent a column in a database table.

    This is a subclass of ``expression.ColumnClause`` and represents an actual
    existing table in the database, in a similar fashion as
    ``TableClause``/``Table``.
    """

    def __init__(self, *args, **kwargs):
        """Construct a new ``Column`` object.

        Arguments are:

        name
          The name of this column.  This should be the identical name as it
          appears, or will appear, in the database.  Name may be omitted at
          construction time but must be assigned before adding a Column
          instance to a Table.

        type\_
          The ``TypeEngine`` for this column.  This can be any subclass of
          ``types.AbstractType``, including the database-agnostic types
          defined in the types module, database-specific types defined within
          specific database modules, or user-defined types. If the column
          contains a ForeignKey, the type can also be None, in which case the
          type assigned will be that of the referenced column.

        \*args
          Constraint, ForeignKey, ColumnDefault and Sequence objects should be
          added as list values.

        \**kwargs
          Keyword arguments include:

          key
            Defaults to the column name: a Python-only *alias name* for this
            column.

            The column will then be identified everywhere in an application,
            including the column list on its Table, by this key, and not the
            given name.  Generated SQL, however, will still reference the
            column by its actual name.

          primary_key
            Defaults to False: True if this column is a primary key column.
            Multiple columns can have this flag set to specify composite
            primary keys.  As an alternative, the primary key of a Table can
            be specified via an explicit ``PrimaryKeyConstraint`` instance
            appended to the Table's list of objects.

          nullable
            Defaults to True : True if this column should allow nulls. True is
            the default unless this column is a primary key column.

          default
            Defaults to None: a scalar, Python callable, or ``ClauseElement``
            representing the *default value* for this column, which will be
            invoked upon insert if this column is not present in the insert
            list or is given a value of None.  The default expression will be
            converted into a ``ColumnDefault`` object upon initialization.

          _is_oid
            Defaults to False: used internally to indicate that this column is
            used as the quasi-hidden "oid" column

          index
            Defaults to False: indicates that this column is indexed. The name
            of the index is autogenerated.  to specify indexes with explicit
            names or indexes that contain multiple columns, use the ``Index``
            construct instead.

          info
            Defaults to {}: A space to store application specific data; this
            must be a dictionary.

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

          autoincrement
            Defaults to True: indicates that integer-based primary key columns
            should have autoincrementing behavior, if supported by the
            underlying database.  This will affect ``CREATE TABLE`` statements
            such that they will use the databases *auto-incrementing* keyword
            (such as ``SERIAL`` for Postgres, ``AUTO_INCREMENT`` for Mysql)
            and will also affect the behavior of some dialects during
            ``INSERT`` statement execution such that they will assume primary
            key values are created in this manner.  If a ``Column`` has an
            explicit ``ColumnDefault`` object (such as via the `default`
            keyword, or a ``Sequence`` or ``PassiveDefault``), then the value
            of `autoincrement` is ignored and is assumed to be False.
            `autoincrement` value is only significant for a column with a type
            or subtype of Integer.

          quote
            When True, indicates that the Column identifier must be quoted.
            This flag does *not* disable quoting; for case-insensitive names,
            use an all lower case identifier.
        """

        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 exceptions.ArgumentError(
                        "May not pass name positionally and as a keyword.")
                name = args.pop(0)
        if args:
            if (isinstance(args[0], types.AbstractType) or
                (isinstance(args[0], type) and
                 issubclass(args[0], types.AbstractType))):
                if type_ is not None:
                    raise exceptions.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._is_oid = kwargs.pop('_is_oid', False)
        self.default = kwargs.pop('default', None)
        self.index = kwargs.pop('index', None)
        self.unique = kwargs.pop('unique', None)
        self.quote = kwargs.pop('quote', False)
        self.onupdate = kwargs.pop('onupdate', None)
        self.autoincrement = kwargs.pop('autoincrement', True)
        self.constraints = util.Set()
        self.foreign_keys = util.OrderedSet()
        util.set_creation_order(self)
        if kwargs.get('info'):
            self._info = kwargs.pop('info')
        if kwargs:
            raise exceptions.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 references the given column via a 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')
        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 exceptions.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 exceptions.ArgumentError("this Column already has a table!")
        if not self._is_oid:
            self._pre_existing_column = table._columns.get(self.key)

            table._columns.replace(self)
        else:
            self._pre_existing_column = None

        if self.primary_key:
            table.primary_key.replace(self)
        elif self.key in table.primary_key:
            raise exceptions.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 exceptions.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 exceptions.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:
            toinit.append(ColumnDefault(self.default))
        if self.onupdate is not None:
            toinit.append(ColumnDefault(self.onupdate, for_update=True))
        self._init_items(*toinit)
        self.args = None

    def copy(self):
        """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, _is_oid = self._is_oid, quote=self.quote, index=self.index, autoincrement=self.autoincrement, *[c.copy() 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, _is_oid = self._is_oid, quote=self.quote, *fk)
        c.table = selectable
        c.proxies = [self]
        c._pre_existing_column = self._pre_existing_column
        if not c._is_oid:
            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 ``Column`` object.

    For a composite (multiple column) FOREIGN KEY, use a ForeignKeyConstraint
    within the Table definition.
    """

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

        column
          A single target column for the key relationship.  A ``Column``
          object or a column name as a string: ``tablename.columnname`` or
          ``schema.tablename.columnname``.

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

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

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

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

        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.

        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._column = None
        self.constraint = constraint
        self.use_alter = use_alter
        self.name = name
        self.onupdate = onupdate
        self.ondelete = ondelete
        self.deferrable = deferrable
        self.initially = initially

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

    def copy(self):
        """Produce a copy of this ForeignKey object."""

        return ForeignKey(self._get_colspec())

    def _get_colspec(self):
        if isinstance(self._colspec, basestring):
            return self._colspec
        elif self._colspec.table.schema is not None:
            return "%s.%s.%s" % (self._colspec.table.schema,
                                 self._colspec.table.name, self._colspec.key)
        else:
            return "%s.%s" % (self._colspec.table.name, 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)

    def column(self):
        # ForeignKey inits its remote column as late as possible, so tables
        # can be defined without dependencies
        if self._column is None:
            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 exceptions.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 exceptions.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 exceptions.NoReferencedTableError(
                        "Could not find table '%s' with which to generate a "
                        "foreign key" % tname)
                table = Table(tname, parenttable.metadata,
                              mustexist=True, schema=schema)
                try:
                    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
                        self._column = table.c[self.parent.key]
                    else:
                        self._column = table.c[colname]
                except KeyError, e:
                    raise exceptions.ArgumentError(
                        "Could not create ForeignKey '%s' on table '%s': "
                        "table '%s' has no column named '%s'" % (
                        self._colspec, parenttable.name, table.name, str(e)))
            
            elif isinstance(self._colspec, expression.Operators):
                self._column = self._colspec.clause_element()
            else:
                self._column = self._colspec

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

    column = property(column)

    def _set_parent(self, column):
        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."""

    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 PassiveDefault(DefaultGenerator):
    """A default that takes effect on the database side."""

    def __init__(self, arg, **kwargs):
        super(PassiveDefault, self).__init__(**kwargs)
        self.arg = arg

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

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 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 exceptions.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."""

    def __init__(self, name, start=None, increment=None, schema=None,
                 optional=False, quote=False, **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 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.
    """

    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 self.columns.contains_column(x)

    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):
        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 exceptions.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):
        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
    ``ForeignKey`` to the definition of a ``Column`` is a shorthand equivalent
    for an unnamed, single column ``ForeignKeyConstraint``.
    """

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

        columns
          A sequence of local column names.  The named columns must be defined
          and present in the parent Table.

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

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

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

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

        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.

        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
        if self.name is None and use_alter:
            raise exceptions.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)
        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):
        return ForeignKeyConstraint([x.parent.name for x in self.elements], [x._get_colspec() 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.
    """

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

    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.
    """

    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 exceptions.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):
        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.
    """

    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 exceptions.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 exceptions.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 [sqlalchemy.schema#Table] objects in the
    collection and their columns may participate in implicit SQL execution.

    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.
    """

    __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 exceptions.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

    # @deprecated
    def connect(self, bind, **kwargs):
        """Bind this MetaData to an Engine.

        Use ``metadata.bind = <engine>`` or ``metadata.bind = <url>``.

        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
    connect = util.deprecated()(connect)

    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):
        self.tables.clear()

    def remove(self, table):
        # TODO: scan all other tables and remove FK _column
        del self.tables[table.key]

    def table_iterator(self, reverse=True, tables=None):
        from sqlalchemy.sql.util import sort_tables
        if tables is None:
            tables = self.tables.values()
        else:
            tables = util.Set(tables).intersection(self.tables.values())
        return iter(sort_tables(tables, reverse=reverse))

    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 ``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 = util.Set(self.tables.keys())

        if only is None:
            load = [name for name in available if name not in current]
        elif 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 exceptions.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 ``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 ``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-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``.

    Use this type of MetaData when your tables are present in more than one
    database and you need to address them simultanesouly.
    """

    __visit_name__ = 'metadata'

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

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

    # @deprecated
    def connect(self, bind, **kwargs):
        """Bind to an Engine in the caller's thread.

        Use ``metadata.bind=<engine>`` or ``metadata.bind=<url>``.

        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)
    connect = util.deprecated()(connect)

    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 ``Connectable``, used by default when ``execute()``
          is invoked without a bind argument.
        """

        if not isinstance(statement, basestring):
            raise exceptions.ArgumentError(
                "Expected a string or unicode SQL statement, got '%r'" %
                statement)
        if (on is not None and
            (not isinstance(on, basestring) and not callable(on))):
            raise exceptions.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
        ``Connectable`` or ``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 ``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 exceptions.ArgumentError(
                "%s does not support DDL events" % type(schema_item).__name__)
        if event not in schema_item.ddl_events:
            raise exceptions.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 exceptions.UnboundExecutionError(msg)
    return bind