path: root/lib/sqlalchemy/schema.py

# schema.py
# Copyright (C) 2005,2006 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.  This means
all the entities within a SQL database that we might want to look at, modify, or create
and delete are described by these objects, in a database-agnostic way.   

A structure of SchemaItems also provides a "visitor" interface which is the primary 
method by which other methods operate upon the schema.  The SQL package extends this
structure with its own clause-specific objects as well as the visitor interface, so that
the schema package "plugs in" to the SQL package.

"""
from sqlalchemy import sql, types, exceptions,util
import sqlalchemy
import copy, re, string

__all__ = ['SchemaItem', 'Table', 'Column', 'ForeignKey', 'Sequence', 'Index', 'ForeignKeyConstraint',
            'PrimaryKeyConstraint', 'CheckConstraint', 'UniqueConstraint',
           'MetaData', 'BoundMetaData', 'DynamicMetaData', 'SchemaVisitor', 'PassiveDefault', 'ColumnDefault']

class SchemaItem(object):
    """base class for items that define a database schema."""
    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 _get_parent(self):
        raise NotImplementedError()
    def _set_parent(self, parent):
        """associate with this SchemaItem's parent object."""
        raise NotImplementedError()
    def __repr__(self):
        return "%s()" % self.__class__.__name__
    def _derived_metadata(self):
        """return the the MetaData to which this item is bound"""
        return None
    def _get_engine(self):
        """return the engine or None if no engine"""
        return self._derived_metadata().engine
    def get_engine(self):
        """return the engine or raise an error if no engine"""
        e = self._get_engine()
        if e is not None:
            return e
        else:
            raise exceptions.InvalidRequestError("This SchemaItem is not connected to any Engine")
        
    def _set_casing_strategy(self, name, kwargs, keyname='case_sensitive'):
        """set the "case_sensitive" argument sent via keywords to the item's constructor.
        
        for the purposes of Table's 'schema' property, the name of the variable is
        optionally configurable."""
        setattr(self, '_%s_setting' % keyname, kwargs.pop(keyname, None))
    def _determine_case_sensitive(self, name, keyname='case_sensitive'):
        """determine the "case_sensitive" value for this item.
        
        for the purposes of Table's 'schema' property, the name of the variable is
        optionally configurable.
        
        a local non-None value overrides all others.  after that, the parent item
        (i.e. Column for a Sequence, Table for a Column, MetaData for a Table) is
        searched for a non-None setting, traversing each parent until none are found.
        finally, case_sensitive is set to True if and only if the name of this item
        is not all lowercase.
        """
        local = getattr(self, '_%s_setting' % keyname, None)
        if local is not None:
            return local
        parent = self
        while parent is not None:
            parent = parent._get_parent()
            if parent is not None:
                parentval = getattr(parent, '_case_sensitive_setting', None)
                if parentval is not None:
                    return parentval
        return name is not None and name.lower() != name
    def _get_case_sensitive(self):
        try:
            return self.__case_sensitive
        except AttributeError:
            self.__case_sensitive = self._determine_case_sensitive(self.name)
            return self.__case_sensitive
    case_sensitive = property(_get_case_sensitive)
        
    engine = property(lambda s:s._get_engine())
    metadata = property(lambda s:s._derived_metadata())
    
def _get_table_key(name, schema):
    if schema is None:
        return name
    else:
        return schema + "." + name
        
class _TableSingleton(type):
    """a metaclass used by the Table object to provide singleton behavior."""
    def __call__(self, name, metadata, *args, **kwargs):
        if isinstance(metadata, sql.Executor):
            # backwards compatibility - get a BoundSchema associated with the engine
            engine = metadata
            if not hasattr(engine, '_legacy_metadata'):
                engine._legacy_metadata = BoundMetaData(engine)
            metadata = engine._legacy_metadata
        elif metadata is not None and not isinstance(metadata, MetaData):
            # they left MetaData out, so assume its another SchemaItem, add it to *args
            args = list(args)
            args.insert(0, metadata)
            metadata = None
            
        if metadata is None:
            metadata = default_metadata
            
        name = str(name)    # in case of incoming unicode
        schema = kwargs.get('schema', None)
        autoload = kwargs.pop('autoload', False)
        autoload_with = kwargs.pop('autoload_with', False)
        mustexist = kwargs.pop('mustexist', False)
        useexisting = kwargs.pop('useexisting', False)
        key = _get_table_key(name, schema)
        try:
            table = metadata.tables[key]
            if len(args):
                if not useexisting:
                    raise exceptions.ArgumentError("Table '%s.%s' is already defined for this MetaData instance." % (schema, name))
            return table
        except KeyError:
            if mustexist:
                raise exceptions.ArgumentError("Table '%s.%s' not defined" % (schema, name))
            table = type.__call__(self, name, metadata, **kwargs)
            table._set_parent(metadata)
            # 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:
                try:
                    if autoload_with:
                        autoload_with.reflecttable(table)
                    else:
                        metadata.get_engine().reflecttable(table)
                except exceptions.NoSuchTableError:
                    del metadata.tables[key]
                    raise
            # initialize all the column, etc. objects.  done after
            # reflection to allow user-overrides
            table._init_items(*args)
            return table

        
class Table(SchemaItem, sql._TableClause):
    """represents a relational database table.  This subclasses sql.TableClause to provide
    a table that is "wired" to an engine.  Whereas TableClause represents a table as its 
    used in a SQL expression, Table represents a table as its created in the database.  
    
    Be sure to look at sqlalchemy.sql.TableImpl for additional methods defined on a Table."""
    __metaclass__ = _TableSingleton
    
    def __init__(self, name, metadata, **kwargs):
        """Construct a Table.
        
        Table objects can be constructed directly.  The init method is actually called via 
        the TableSingleton metaclass.  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 : options include:
        
        schema=None : 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.
        
        autoload=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.
        
        redefine=False : if this Table has already been defined in the application, clear out its columns
        and redefine with new arguments.
        
        mustexist=False : indicates that this Table must already have been defined elsewhere in the application,
        else an exception is raised.
        
        useexisting=False : indicates that if this Table was already defined elsewhere in the application, disregard
        the rest of the constructor arguments.  If this flag and the "redefine" flag are not set, constructing 
        the same table twice will result in an exception.
        
        owner=None : optional owning user of this table.  useful for databases such as Oracle to aid in table
        reflection.
        
        quote=False : indicates that the Table identifier must be properly escaped and quoted before being sent 
        to the database. This flag overrides all other quoting behavior.
        
        quote_schema=False : indicates that the Namespace identifier must be properly escaped and quoted before being sent 
        to the database. This flag overrides all other quoting behavior.
        
        case_sensitive=True : indicates that the identifier should be interpreted by the database in the natural case for identifiers.
        Mixed case is not sufficient to cause this identifier to be quoted; it must contain an illegal character.
        
        case_sensitive_schema=True : indicates that the identifier should be interpreted by the database in the natural case for identifiers.
        Mixed case is not sufficient to cause this identifier to be quoted; it must contain an illegal character.
        """
        super(Table, self).__init__(name)
        self._metadata = metadata
        self.schema = kwargs.pop('schema', None)
        self.indexes = util.Set()
        self.constraints = util.Set()
        self.primary_key = PrimaryKeyConstraint()
        self.quote = kwargs.get('quote', False)
        self.quote_schema = kwargs.get('quote_schema', False)
        if self.schema is not None:
            self.fullname = "%s.%s" % (self.schema, self.name)
        else:
            self.fullname = self.name
        self.owner = kwargs.pop('owner', None)

        self._set_casing_strategy(name, kwargs)
        self._set_casing_strategy(self.schema or '', kwargs, keyname='case_sensitive_schema')
        self.kwargs = kwargs

    def _get_case_sensitive_schema(self):
        try:
            return getattr(self, '_case_sensitive_schema')
        except AttributeError:
            setattr(self, '_case_sensitive_schema', self._determine_case_sensitive(self.schema or '', keyname='case_sensitive_schema'))
            return getattr(self, '_case_sensitive_schema')
    case_sensitive_schema = property(_get_case_sensitive_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)
    primary_key = property(lambda s:s._primary_key, _set_primary_key)
    
    def _derived_metadata(self):
        return self._metadata
    def __repr__(self):
        return "Table(%s)" % string.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']]
       , ',\n')
    
    def __str__(self):
        return _get_table_key(self.name, 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 _get_parent(self):
        return self._metadata    
    def _set_parent(self, metadata):
        metadata.tables[_get_table_key(self.name, self.schema)] = self
        self._metadata = metadata
    def accept_schema_visitor(self, visitor, traverse=True): 
        if traverse:
            for c in self.columns:
                c.accept_schema_visitor(visitor, True)
        return visitor.visit_table(self)

    def exists(self, connectable=None):
        """return True if this table exists."""
        if connectable is None:
            connectable = self.get_engine()

        def do(conn):
            e = conn.engine
            return e.dialect.has_table(conn, self.name)
        return connectable.run_callable(do)

    def create(self, connectable=None, checkfirst=False):
        """issue a CREATE statement for this table.
        
        see also metadata.create_all()."""
        self.metadata.create_all(connectable=connectable, checkfirst=checkfirst, tables=[self])
    def drop(self, connectable=None, checkfirst=False):
        """issue a DROP statement for this table.
        
        see also metadata.drop_all()."""
        self.metadata.drop_all(connectable=connectable, 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, sql._ColumnClause):
    """represents a column in a database table.  this is a subclass of sql.ColumnClause and
    represents an actual existing table in the database, in a similar fashion as TableClause/Table."""
    def __init__(self, name, type, *args, **kwargs):
        """constructs 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.
        
        type : this is the type of column. This can be any subclass of types.TypeEngine,
        including the database-agnostic types defined in the types module, database-specific types
        defined within specific database modules, or user-defined types.
        
        *args : ForeignKey and Sequence objects should be added as list values.
        
        **kwargs : keyword arguments include:
        
        key=None : an optional "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=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=True : True if this column should allow nulls. Defaults to True unless this column is a primary
        key column.
        
        default=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.

        hidden=False : indicates this column should not be listed in the
        table's list of columns.  Used for the "oid" column, which generally
        isnt in column lists.

        index=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.

        unique=False : Indicates that this column 
        contains a unique constraint, or if index=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=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=False : indicates that the Column identifier must be properly escaped and quoted before being sent 
        to the database.  This flag should normally not be required as dialects can auto-detect conditions where quoting
        is required.

        case_sensitive=True : indicates that the identifier should be interpreted by the database in the natural case for identifiers.
        Mixed case is not sufficient to cause this identifier to be quoted; it must contain an illegal character.
        """
        name = str(name) # in case of incoming unicode
        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.hidden = kwargs.pop('hidden', 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._set_casing_strategy(name, kwargs)
        self.onupdate = kwargs.pop('onupdate', None)
        self.autoincrement = kwargs.pop('autoincrement', True)
        self.constraints = util.Set()
        self.__originating_column = self
        self._foreign_keys = util.Set()
        if len(kwargs):
            raise exceptions.ArgumentError("Unknown arguments passed to Column: " + repr(kwargs.keys()))

    primary_key = util.SimpleProperty('_primary_key')
    foreign_keys = util.SimpleProperty('_foreign_keys')
    columns = property(lambda self:[self])

    def __str__(self):
        if self.table is not None:
            if self.table.named_with_column():
                return self.table.name + "." + self.name
            else:
                return self.name
            return self.name
        else:
            return self.name
    
    def _derived_metadata(self):
        return self.table.metadata
    def _get_engine(self):
        return self.table.engine
    
    def append_foreign_key(self, fk):
        fk._set_parent(self)
            
    def __repr__(self):
       return "Column(%s)" % string.join(
        [repr(self.name)] + [repr(self.type)] +
        [repr(x) for x in self.foreign_keys if x is not None] +
        ["%s=%s" % (k, repr(getattr(self, k))) for k in ['key', 'primary_key', 'nullable', 'hidden', 'default', 'onupdate']]
       , ',')
        
    def _get_parent(self):
        return self.table        

    def _set_parent(self, table):
        if getattr(self, 'table', None) is not None:
            raise exceptions.ArgumentError("this Column already has a table!")
        if not self.hidden:
            table._columns.add(self)
        if self.primary_key:
            table.primary_key.add(self)
        self.table = table

        if self.index:
            if isinstance(self.index, str):
                raise exceptions.ArgumentError("The 'index' keyword argument on Column is boolean only.  To create indexes with a specific name, append an explicit Index object to the Table's list of elements.")
            Index('ix_%s' % self._label, self, unique=self.unique)
        elif self.unique:
            if isinstance(self.unique, str):
                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 or Index object to the Table's list of elements.")
            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): 
        """creates 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, hidden = self.hidden, case_sensitive=self._case_sensitive_setting, quote=self.quote)
        
    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, hidden = self.hidden, quote=self.quote, *fk)
        c.table = selectable
        c.orig_set = self.orig_set
        c.__originating_column = self.__originating_column
        if not c.hidden:
            selectable.columns.add(c)
            if self.primary_key:
                selectable.primary_key.add(c)
        [c._init_items(f) for f in fk]
        return c

    def _case_sens(self):
        """redirect the 'case_sensitive' accessor to use the ultimate parent column which created
        this one."""
        return self.__originating_column._get_case_sensitive()
    case_sensitive = property(_case_sens)
    
    def accept_schema_visitor(self, visitor, traverse=True):
        """traverses the given visitor to this Column's default and foreign key object,
        then calls visit_column on the visitor."""
        if traverse:
            if self.default is not None:
                self.default.accept_schema_visitor(visitor, traverse=True)
            if self.onupdate is not None:
                self.onupdate.accept_schema_visitor(visitor, traverse=True)
            for f in self.foreign_keys:
                f.accept_schema_visitor(visitor, traverse=True)
            for constraint in self.constraints:
                constraint.accept_schema_visitor(visitor, traverse=True)
        visitor.visit_column(self)


class ForeignKey(SchemaItem):
    """defines a column-level ForeignKey constraint between two columns.  
    
    ForeignKey is specified as an argument to a Column object.
    
    One or more ForeignKey objects are used within a ForeignKeyConstraint
    object which represents the table-level constraint definition."""
    def __init__(self, column, constraint=None, use_alter=False, name=None):
        """Construct a new ForeignKey object.  
        
        "column" can be a schema.Column object representing the relationship, 
        or just its string name given as "tablename.columnname".  schema can be 
        specified as "schema.tablename.columnname" 
        
        "constraint" is the owning ForeignKeyConstraint object, if any.  if not given,
        then a ForeignKeyConstraint will be automatically created and added to the parent table.
        """
        if isinstance(column, unicode):
            column = str(column)
        self._colspec = column
        self._column = None
        self.constraint = constraint
        self.use_alter = use_alter
        self.name = name
        
    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, str):
            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)
        
    def references(self, table):
        """returns True if the given table is referenced by this ForeignKey."""
        return table.corresponding_column(self.column, False) is not None
        
    def _init_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, str):
                # 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.orig_set:
                    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"^([\w_-]+)(?:\.([\w_-]+))?(?:\.([\w_-]+))?$", self._colspec)
                if m is None:
                    raise exceptions.ArgumentError("Invalid foreign key column specification: " + self._colspec)
                if m.group(3) is None:
                    (tname, colname) = m.group(1, 2)
                    schema = parenttable.schema
                else:
                    (schema,tname,colname) = m.group(1,2,3)
                table = Table(tname, parenttable.metadata, mustexist=True, schema=schema)
                try:
                    if colname is None:
                        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, e.args[0]))
            else:
                self._column = self._colspec

        return self._column
            
    column = property(lambda s: s._init_column())

    def accept_schema_visitor(self, visitor, traverse=True):
        """calls the visit_foreign_key method on the given visitor."""
        visitor.visit_foreign_key(self)
  
    def _get_parent(self):
        return self.parent
    def _set_parent(self, column):
        self.parent = column

        if self.constraint is None and isinstance(self.parent.table, Table):
            self.constraint = ForeignKeyConstraint([],[], use_alter=self.use_alter, name=self.name)
            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 = metadata
    def _derived_metadata(self):
        try:
            return self.column.table.metadata
        except AttributeError:
            return self._metadata
    def _get_parent(self):
        return getattr(self, 'column', None)
    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, **kwargs):
        return self.get_engine().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 accept_schema_visitor(self, visitor, traverse=True):
        return visitor.visit_passive_default(self)
    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)
        self.arg = arg
    def accept_schema_visitor(self, visitor, traverse=True):
        """calls the visit_column_default method on the given visitor."""
        if self.for_update:
            return visitor.visit_column_onupdate(self)
        else:
            return visitor.visit_column_default(self)
    def __repr__(self):
        return "ColumnDefault(%s)" % repr(self.arg)
        
class Sequence(DefaultGenerator):
    """represents a sequence, which applies to Oracle and Postgres databases."""
    def __init__(self, name, start = None, increment = 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._set_casing_strategy(name, kwargs)
    def __repr__(self):
        return "Sequence(%s)" % string.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):
       self.get_engine().create(self)
       return self
    def drop(self):
       self.get_engine().drop(self)
    def accept_schema_visitor(self, visitor, traverse=True):
        """calls the visit_seauence method on the given visitor."""
        return visitor.visit_sequence(self)

class Constraint(SchemaItem):
    """represents a table-level Constraint such as a composite primary key, foreign key, or unique constraint.
    
    Implements a hybrid of dict/setlike behavior with regards to the list of underying columns"""
    def __init__(self, name=None):
        self.name = name
        self.columns = sql.ColumnCollection()
    def __contains__(self, x):
        return x in self.columns
    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()
    def _get_parent(self):
        return getattr(self, 'table', None)

class CheckConstraint(Constraint):
    def __init__(self, sqltext, name=None):
        super(CheckConstraint, self).__init__(name)
        self.sqltext = sqltext
    def accept_schema_visitor(self, visitor, traverse=True):
        visitor.visit_check_constraint(self)
    def _set_parent(self, parent):
        self.parent = parent
        parent.constraints.add(self)
                
class ForeignKeyConstraint(Constraint):
    """table-level foreign key constraint, represents a colleciton of ForeignKey objects."""
    def __init__(self, columns, refcolumns, name=None, onupdate=None, ondelete=None, use_alter=False):
        super(ForeignKeyConstraint, self).__init__(name)
        self.__colnames = columns
        self.__refcolnames = refcolumns
        self.elements = util.Set()
        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
        table.constraints.add(self)
        for (c, r) in zip(self.__colnames, self.__refcolnames):
            self.append_element(c,r)
    def accept_schema_visitor(self, visitor, traverse=True):
        visitor.visit_foreign_key_constraint(self)
    def append_element(self, col, refcol):
        fk = ForeignKey(refcol, constraint=self)
        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)
                        
class PrimaryKeyConstraint(Constraint):
    def __init__(self, *columns, **kwargs):
        super(PrimaryKeyConstraint, self).__init__(name=kwargs.pop('name', None))
        self.__colnames = list(columns)
    def _set_parent(self, table):
        self.table = table
        table.primary_key = self
        for c in self.__colnames:
            self.append_column(table.c[c])
    def accept_schema_visitor(self, visitor, traverse=True):
        visitor.visit_primary_key_constraint(self)
    def add(self, col):
        self.append_column(col)
    def append_column(self, col):
        self.columns.add(col)
        col.primary_key=True
    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):
    def __init__(self, *columns, **kwargs):
        super(UniqueConstraint, self).__init__(name=kwargs.pop('name', None))
        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 accept_schema_visitor(self, visitor, traverse=True):
        visitor.visit_unique_constraint(self)
        
class Index(SchemaItem):
    """Represents an index of columns from a database table
    """
    def __init__(self, name, *columns, **kwargs):
        """Constructs 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.

        **kw : keyword arguments include:

        unique=True : create a unique index
        """
        self.name = name
        self.columns = []
        self.table = None
        self.unique = kwargs.pop('unique', False)
        self._init_items(*columns)

    def _derived_metadata(self):
        return self.table.metadata
    def _init_items(self, *args):
        for column in args:
            self.append_column(column)
    def _get_parent(self):
        return self.table    
    def _set_parent(self, table):
        self.table = table
        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, connectable=None):
        if connectable is not None:
            connectable.create(self)
        else:
            self.get_engine().create(self)
        return self
    def drop(self, connectable=None):
        if connectable is not None:
            connectable.drop(self)
        else:
            self.get_engine().drop(self)
    def accept_schema_visitor(self, visitor, traverse=True):
        visitor.visit_index(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):
    """represents a collection of Tables and their associated schema constructs."""
    def __init__(self, name=None, **kwargs):
        self.tables = {}
        self.name = name
        self._set_casing_strategy(name, kwargs)
    def is_bound(self):
        return False
    def clear(self):
        self.tables.clear()

    def table_iterator(self, reverse=True, tables=None):
        import sqlalchemy.sql_util
        if tables is None:
            tables = self.tables.values()
        else:
            tables = util.Set(tables).intersection(self.tables.values())
        sorter = sqlalchemy.sql_util.TableCollection(list(tables))
        return iter(sorter.sort(reverse=reverse))
    def _get_parent(self):
        return None    
    def create_all(self, connectable=None, tables=None, checkfirst=True):
        """create all tables stored in this metadata.
        
        This will conditionally create tables depending on if they do not yet
        exist in the database.
        
        connectable - a Connectable used to access the database; or use the engine
        bound to this MetaData.
        
        tables - optional list of tables, which is a subset of the total
        tables in the MetaData (others are ignored)"""
        if connectable is None:
            connectable = self.get_engine()
        connectable.create(self, checkfirst=checkfirst, tables=tables)
        
    def drop_all(self, connectable=None, tables=None, checkfirst=True):
        """drop all tables stored in this metadata.
        
        This will conditionally drop tables depending on if they currently 
        exist in the database.
        
        connectable - a Connectable used to access the database; or use the engine
        bound to this MetaData.
        
        tables - optional list of tables, which is a subset of the total
        tables in the MetaData (others are ignored)
        """
        if connectable is None:
            connectable = self.get_engine()
        connectable.drop(self, checkfirst=checkfirst, tables=tables)
                
    
    def accept_schema_visitor(self, visitor, traverse=True):
        visitor.visit_metadata(self)
            
    def _derived_metadata(self):
        return self
    def _get_engine(self):
        if not self.is_bound():
            return None
        return self._engine
                
class BoundMetaData(MetaData):
    """builds upon MetaData to provide the capability to bind to an Engine implementation."""
    def __init__(self, engine_or_url, name=None, **kwargs):
        super(BoundMetaData, self).__init__(name, **kwargs)
        if isinstance(engine_or_url, str):
            self._engine = sqlalchemy.create_engine(engine_or_url, **kwargs)
        else:
            self._engine = engine_or_url
    def is_bound(self):
        return True

class DynamicMetaData(MetaData):
    """builds upon MetaData to provide the capability to bind to multiple Engine implementations
    on a dynamically alterable, thread-local basis."""
    def __init__(self, name=None, threadlocal=True, **kwargs):
        super(DynamicMetaData, self).__init__(name, **kwargs)
        if threadlocal:
            self.context = util.ThreadLocal()
        else:
            self.context = self
        self.__engines = {}
    def connect(self, engine_or_url, **kwargs):
        if isinstance(engine_or_url, str):
            try:
                self.context._engine = self.__engines[engine_or_url]
            except KeyError:
                e = sqlalchemy.create_engine(engine_or_url, **kwargs)
                self.__engines[engine_or_url] = e
                self.context._engine = e
        else:
            if not self.__engines.has_key(engine_or_url):
                self.__engines[engine_or_url] = engine_or_url
            self.context._engine = engine_or_url
    def is_bound(self):
        return hasattr(self.context, '_engine') and self.context._engine is not None
    def dispose(self):
        """disposes all Engines to which this DynamicMetaData has been connected."""
        for e in self.__engines.values():
            e.dispose()
    def _get_engine(self):
        if hasattr(self.context, '_engine'):
            return self.context._engine
        else:
            return None
    engine=property(_get_engine)
            
class SchemaVisitor(sql.ClauseVisitor):
    """defines the visiting for SchemaItem objects"""
    def visit_schema(self, schema):
        """visit a generic SchemaItem"""
        pass
    def visit_table(self, table):
        """visit a Table."""
        pass
    def visit_column(self, column):
        """visit a Column."""
        pass
    def visit_foreign_key(self, join):
        """visit a ForeignKey."""
        pass
    def visit_index(self, index):
        """visit an Index."""
        pass
    def visit_passive_default(self, default):
        """visit a passive default"""
        pass
    def visit_column_default(self, default):
        """visit a ColumnDefault."""
        pass
    def visit_column_onupdate(self, onupdate):
        """visit a ColumnDefault with the "for_update" flag set."""
        pass
    def visit_sequence(self, sequence):
        """visit a Sequence."""
        pass
    def visit_primary_key_constraint(self, constraint):
        pass
    def visit_foreign_key_constraint(self, constraint):
        pass
    def visit_unique_constraint(self, constraint):
        pass
    def visit_check_constraint(self, constraint):
        pass
        
default_metadata = DynamicMetaData('default')