- reorganization of declarative such that file sizes are managable again.

the vast majority of file lines are spent on documentation, which moves
into package __init__.  The core declarative idea lives in base and
is back down to its originally low size of under 500 lines.  The various
helpers and such move into api.py, and the full span of string lookup
moves into a new module clsregistry.  the rest of declarative only
refers to two functions in clsregistry in three places inside of base.
- [feature] Declarative now maintains a registry
of classes by string name as well as by full
module-qualified name.   Multiple classes with the
same name can now be looked up based on a module-qualified
string within relationship().   Simple class name
lookups where more than one class shares the same
name now raises an informative error message.
[ticket:2338]
- lots of tests to ensure the new weak referencing memory management
is maintained by the new class registry system.   this ticket was
served very well by waiting to do #2526 first, else this would
have needed to be rewritten anyway.

1 files changed, 1065 insertions, 0 deletions

diff --git a/lib/sqlalchemy/ext/declarative/__init__.py b/lib/sqlalchemy/ext/declarative/__init__.py
new file mode 100644
index 000000000..e6d6e388b
--- /dev/null
+++ b/lib/sqlalchemy/ext/declarative/__init__.py
@@ -0,0 +1,1065 @@
+# ext/declarative/__init__.py
+# Copyright (C) 2005-2012 the SQLAlchemy authors and contributors <see AUTHORS file>
+#
+# This module is part of SQLAlchemy and is released under
+# the MIT License: http://www.opensource.org/licenses/mit-license.php
+
+"""
+Synopsis
+========
+
+SQLAlchemy object-relational configuration involves the
+combination of :class:`.Table`, :func:`.mapper`, and class
+objects to define a mapped class.
+:mod:`~sqlalchemy.ext.declarative` allows all three to be
+expressed at once within the class declaration. As much as
+possible, regular SQLAlchemy schema and ORM constructs are
+used directly, so that configuration between "classical" ORM
+usage and declarative remain highly similar.
+
+As a simple example::
+
+    from sqlalchemy.ext.declarative import declarative_base
+
+    Base = declarative_base()
+
+    class SomeClass(Base):
+        __tablename__ = 'some_table'
+        id = Column(Integer, primary_key=True)
+        name =  Column(String(50))
+
+Above, the :func:`declarative_base` callable returns a new base class from
+which all mapped classes should inherit. When the class definition is
+completed, a new :class:`.Table` and
+:func:`.mapper` will have been generated.
+
+The resulting table and mapper are accessible via
+``__table__`` and ``__mapper__`` attributes on the
+``SomeClass`` class::
+
+    # access the mapped Table
+    SomeClass.__table__
+
+    # access the Mapper
+    SomeClass.__mapper__
+
+Defining Attributes
+===================
+
+In the previous example, the :class:`.Column` objects are
+automatically named with the name of the attribute to which they are
+assigned.
+
+To name columns explicitly with a name distinct from their mapped attribute,
+just give the column a name.  Below, column "some_table_id" is mapped to the
+"id" attribute of `SomeClass`, but in SQL will be represented as "some_table_id"::
+
+    class SomeClass(Base):
+        __tablename__ = 'some_table'
+        id = Column("some_table_id", Integer, primary_key=True)
+
+Attributes may be added to the class after its construction, and they will be
+added to the underlying :class:`.Table` and
+:func:`.mapper()` definitions as appropriate::
+
+    SomeClass.data = Column('data', Unicode)
+    SomeClass.related = relationship(RelatedInfo)
+
+Classes which are constructed using declarative can interact freely
+with classes that are mapped explicitly with :func:`mapper`.
+
+It is recommended, though not required, that all tables
+share the same underlying :class:`~sqlalchemy.schema.MetaData` object,
+so that string-configured :class:`~sqlalchemy.schema.ForeignKey`
+references can be resolved without issue.
+
+Accessing the MetaData
+=======================
+
+The :func:`declarative_base` base class contains a
+:class:`.MetaData` object where newly defined
+:class:`.Table` objects are collected. This object is
+intended to be accessed directly for
+:class:`.MetaData`-specific operations. Such as, to issue
+CREATE statements for all tables::
+
+    engine = create_engine('sqlite://')
+    Base.metadata.create_all(engine)
+
+The usual techniques of associating :class:`.MetaData:` with :class:`.Engine`
+apply, such as assigning to the ``bind`` attribute::
+
+    Base.metadata.bind = create_engine('sqlite://')
+
+To associate the engine with the :func:`declarative_base` at time
+of construction, the ``bind`` argument is accepted::
+
+    Base = declarative_base(bind=create_engine('sqlite://'))
+
+:func:`declarative_base` can also receive a pre-existing
+:class:`.MetaData` object, which allows a
+declarative setup to be associated with an already
+existing traditional collection of :class:`~sqlalchemy.schema.Table`
+objects::
+
+    mymetadata = MetaData()
+    Base = declarative_base(metadata=mymetadata)
+
+Configuring Relationships
+=========================
+
+Relationships to other classes are done in the usual way, with the added
+feature that the class specified to :func:`~sqlalchemy.orm.relationship`
+may be a string name.  The "class registry" associated with ``Base``
+is used at mapper compilation time to resolve the name into the actual
+class object, which is expected to have been defined once the mapper
+configuration is used::
+
+    class User(Base):
+        __tablename__ = 'users'
+
+        id = Column(Integer, primary_key=True)
+        name = Column(String(50))
+        addresses = relationship("Address", backref="user")
+
+    class Address(Base):
+        __tablename__ = 'addresses'
+
+        id = Column(Integer, primary_key=True)
+        email = Column(String(50))
+        user_id = Column(Integer, ForeignKey('users.id'))
+
+Column constructs, since they are just that, are immediately usable,
+as below where we define a primary join condition on the ``Address``
+class using them::
+
+    class Address(Base):
+        __tablename__ = 'addresses'
+
+        id = Column(Integer, primary_key=True)
+        email = Column(String(50))
+        user_id = Column(Integer, ForeignKey('users.id'))
+        user = relationship(User, primaryjoin=user_id == User.id)
+
+In addition to the main argument for :func:`~sqlalchemy.orm.relationship`,
+other arguments which depend upon the columns present on an as-yet
+undefined class may also be specified as strings.  These strings are
+evaluated as Python expressions.  The full namespace available within
+this evaluation includes all classes mapped for this declarative base,
+as well as the contents of the ``sqlalchemy`` package, including
+expression functions like :func:`~sqlalchemy.sql.expression.desc` and
+:attr:`~sqlalchemy.sql.expression.func`::
+
+    class User(Base):
+        # ....
+        addresses = relationship("Address",
+                             order_by="desc(Address.email)",
+                             primaryjoin="Address.user_id==User.id")
+
+For the case where more than one module contains a class of the same name,
+string class names can also be specified as fully module-qualified paths
+within any of these string expressions::
+
+    class User(Base):
+        # ....
+        addresses = relationship("myapp.model.address.Address",
+                             order_by="desc(myapp.model.address.Address.email)",
+                             primaryjoin="myapp.model.address.Address.user_id=="
+                                            "myapp.model.user.User.id")
+
+.. versionadded:: 0.8
+   Fully module-qualified paths can be used when specifying string arguments
+   with Declarative.
+
+Two alternatives also exist to using string-based attributes.  A lambda
+can also be used, which will be evaluated after all mappers have been
+configured::
+
+    class User(Base):
+        # ...
+        addresses = relationship(lambda: Address,
+                             order_by=lambda: desc(Address.email),
+                             primaryjoin=lambda: Address.user_id==User.id)
+
+Or, the relationship can be added to the class explicitly after the classes
+are available::
+
+    User.addresses = relationship(Address,
+                              primaryjoin=Address.user_id==User.id)
+
+
+
+
+Configuring Many-to-Many Relationships
+======================================
+
+Many-to-many relationships are also declared in the same way
+with declarative as with traditional mappings. The
+``secondary`` argument to
+:func:`.relationship` is as usual passed a
+:class:`.Table` object, which is typically declared in the
+traditional way.  The :class:`.Table` usually shares
+the :class:`.MetaData` object used by the declarative base::
+
+    keywords = Table(
+        'keywords', Base.metadata,
+        Column('author_id', Integer, ForeignKey('authors.id')),
+        Column('keyword_id', Integer, ForeignKey('keywords.id'))
+        )
+
+    class Author(Base):
+        __tablename__ = 'authors'
+        id = Column(Integer, primary_key=True)
+        keywords = relationship("Keyword", secondary=keywords)
+
+Like other :func:`.relationship` arguments, a string is accepted as well,
+passing the string name of the table as defined in the ``Base.metadata.tables``
+collection::
+
+    class Author(Base):
+        __tablename__ = 'authors'
+        id = Column(Integer, primary_key=True)
+        keywords = relationship("Keyword", secondary="keywords")
+
+As with traditional mapping, its generally not a good idea to use
+a :class:`.Table` as the "secondary" argument which is also mapped to
+a class, unless the :class:`.relationship` is declared with ``viewonly=True``.
+Otherwise, the unit-of-work system may attempt duplicate INSERT and
+DELETE statements against the underlying table.
+
+.. _declarative_sql_expressions:
+
+Defining SQL Expressions
+========================
+
+See :ref:`mapper_sql_expressions` for examples on declaratively
+mapping attributes to SQL expressions.
+
+.. _declarative_table_args:
+
+Table Configuration
+===================
+
+Table arguments other than the name, metadata, and mapped Column
+arguments are specified using the ``__table_args__`` class attribute.
+This attribute accommodates both positional as well as keyword
+arguments that are normally sent to the
+:class:`~sqlalchemy.schema.Table` constructor.
+The attribute can be specified in one of two forms. One is as a
+dictionary::
+
+    class MyClass(Base):
+        __tablename__ = 'sometable'
+        __table_args__ = {'mysql_engine':'InnoDB'}
+
+The other, a tuple, where each argument is positional
+(usually constraints)::
+
+    class MyClass(Base):
+        __tablename__ = 'sometable'
+        __table_args__ = (
+                ForeignKeyConstraint(['id'], ['remote_table.id']),
+                UniqueConstraint('foo'),
+                )
+
+Keyword arguments can be specified with the above form by
+specifying the last argument as a dictionary::
+
+    class MyClass(Base):
+        __tablename__ = 'sometable'
+        __table_args__ = (
+                ForeignKeyConstraint(['id'], ['remote_table.id']),
+                UniqueConstraint('foo'),
+                {'autoload':True}
+                )
+
+Using a Hybrid Approach with __table__
+=======================================
+
+As an alternative to ``__tablename__``, a direct
+:class:`~sqlalchemy.schema.Table` construct may be used.  The
+:class:`~sqlalchemy.schema.Column` objects, which in this case require
+their names, will be added to the mapping just like a regular mapping
+to a table::
+
+    class MyClass(Base):
+        __table__ = Table('my_table', Base.metadata,
+            Column('id', Integer, primary_key=True),
+            Column('name', String(50))
+        )
+
+``__table__`` provides a more focused point of control for establishing
+table metadata, while still getting most of the benefits of using declarative.
+An application that uses reflection might want to load table metadata elsewhere
+and pass it to declarative classes::
+
+    from sqlalchemy.ext.declarative import declarative_base
+
+    Base = declarative_base()
+    Base.metadata.reflect(some_engine)
+
+    class User(Base):
+        __table__ = metadata.tables['user']
+
+    class Address(Base):
+        __table__ = metadata.tables['address']
+
+Some configuration schemes may find it more appropriate to use ``__table__``,
+such as those which already take advantage of the data-driven nature of
+:class:`.Table` to customize and/or automate schema definition.
+
+Note that when the ``__table__`` approach is used, the object is immediately
+usable as a plain :class:`.Table` within the class declaration body itself,
+as a Python class is only another syntactical block.  Below this is illustrated
+by using the ``id`` column in the ``primaryjoin`` condition of a :func:`.relationship`::
+
+    class MyClass(Base):
+        __table__ = Table('my_table', Base.metadata,
+            Column('id', Integer, primary_key=True),
+            Column('name', String(50))
+        )
+
+        widgets = relationship(Widget,
+                    primaryjoin=Widget.myclass_id==__table__.c.id)
+
+Similarly, mapped attributes which refer to ``__table__`` can be placed inline,
+as below where we assign the ``name`` column to the attribute ``_name``, generating
+a synonym for ``name``::
+
+    from sqlalchemy.ext.declarative import synonym_for
+
+    class MyClass(Base):
+        __table__ = Table('my_table', Base.metadata,
+            Column('id', Integer, primary_key=True),
+            Column('name', String(50))
+        )
+
+        _name = __table__.c.name
+
+        @synonym_for("_name")
+        def name(self):
+            return "Name: %s" % _name
+
+Using Reflection with Declarative
+=================================
+
+It's easy to set up a :class:`.Table` that uses ``autoload=True``
+in conjunction with a mapped class::
+
+    class MyClass(Base):
+        __table__ = Table('mytable', Base.metadata,
+                        autoload=True, autoload_with=some_engine)
+
+However, one improvement that can be made here is to not
+require the :class:`.Engine` to be available when classes are
+being first declared.   To achieve this, use the
+:class:`.DeferredReflection` mixin, which sets up mappings
+only after a special ``prepare(engine)`` step is called::
+
+    from sqlalchemy.ext.declarative import declarative_base, DeferredReflection
+
+    Base = declarative_base(cls=DeferredReflection)
+
+    class Foo(Base):
+        __tablename__ = 'foo'
+        bars = relationship("Bar")
+
+    class Bar(Base):
+        __tablename__ = 'bar'
+
+        # illustrate overriding of "bar.foo_id" to have
+        # a foreign key constraint otherwise not
+        # reflected, such as when using MySQL
+        foo_id = Column(Integer, ForeignKey('foo.id'))
+
+    Base.prepare(e)
+
+.. versionadded:: 0.8
+   Added :class:`.DeferredReflection`.
+
+Mapper Configuration
+====================
+
+Declarative makes use of the :func:`~.orm.mapper` function internally
+when it creates the mapping to the declared table.   The options
+for :func:`~.orm.mapper` are passed directly through via the ``__mapper_args__``
+class attribute.  As always, arguments which reference locally
+mapped columns can reference them directly from within the
+class declaration::
+
+    from datetime import datetime
+
+    class Widget(Base):
+        __tablename__ = 'widgets'
+
+        id = Column(Integer, primary_key=True)
+        timestamp = Column(DateTime, nullable=False)
+
+        __mapper_args__ = {
+                        'version_id_col': timestamp,
+                        'version_id_generator': lambda v:datetime.now()
+                    }
+
+.. _declarative_inheritance:
+
+Inheritance Configuration
+=========================
+
+Declarative supports all three forms of inheritance as intuitively
+as possible.  The ``inherits`` mapper keyword argument is not needed
+as declarative will determine this from the class itself.   The various
+"polymorphic" keyword arguments are specified using ``__mapper_args__``.
+
+Joined Table Inheritance
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Joined table inheritance is defined as a subclass that defines its own
+table::
+
+    class Person(Base):
+        __tablename__ = 'people'
+        id = Column(Integer, primary_key=True)
+        discriminator = Column('type', String(50))
+        __mapper_args__ = {'polymorphic_on': discriminator}
+
+    class Engineer(Person):
+        __tablename__ = 'engineers'
+        __mapper_args__ = {'polymorphic_identity': 'engineer'}
+        id = Column(Integer, ForeignKey('people.id'), primary_key=True)
+        primary_language = Column(String(50))
+
+Note that above, the ``Engineer.id`` attribute, since it shares the
+same attribute name as the ``Person.id`` attribute, will in fact
+represent the ``people.id`` and ``engineers.id`` columns together,
+with the "Engineer.id" column taking precedence if queried directly.
+To provide the ``Engineer`` class with an attribute that represents
+only the ``engineers.id`` column, give it a different attribute name::
+
+    class Engineer(Person):
+        __tablename__ = 'engineers'
+        __mapper_args__ = {'polymorphic_identity': 'engineer'}
+        engineer_id = Column('id', Integer, ForeignKey('people.id'),
+                                                    primary_key=True)
+        primary_language = Column(String(50))
+
+
+.. versionchanged:: 0.7 joined table inheritance favors the subclass
+   column over that of the superclass, such as querying above
+   for ``Engineer.id``.  Prior to 0.7 this was the reverse.
+
+Single Table Inheritance
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Single table inheritance is defined as a subclass that does not have
+its own table; you just leave out the ``__table__`` and ``__tablename__``
+attributes::
+
+    class Person(Base):
+        __tablename__ = 'people'
+        id = Column(Integer, primary_key=True)
+        discriminator = Column('type', String(50))
+        __mapper_args__ = {'polymorphic_on': discriminator}
+
+    class Engineer(Person):
+        __mapper_args__ = {'polymorphic_identity': 'engineer'}
+        primary_language = Column(String(50))
+
+When the above mappers are configured, the ``Person`` class is mapped
+to the ``people`` table *before* the ``primary_language`` column is
+defined, and this column will not be included in its own mapping.
+When ``Engineer`` then defines the ``primary_language`` column, the
+column is added to the ``people`` table so that it is included in the
+mapping for ``Engineer`` and is also part of the table's full set of
+columns.  Columns which are not mapped to ``Person`` are also excluded
+from any other single or joined inheriting classes using the
+``exclude_properties`` mapper argument.  Below, ``Manager`` will have
+all the attributes of ``Person`` and ``Manager`` but *not* the
+``primary_language`` attribute of ``Engineer``::
+
+    class Manager(Person):
+        __mapper_args__ = {'polymorphic_identity': 'manager'}
+        golf_swing = Column(String(50))
+
+The attribute exclusion logic is provided by the
+``exclude_properties`` mapper argument, and declarative's default
+behavior can be disabled by passing an explicit ``exclude_properties``
+collection (empty or otherwise) to the ``__mapper_args__``.
+
+Concrete Table Inheritance
+~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Concrete is defined as a subclass which has its own table and sets the
+``concrete`` keyword argument to ``True``::
+
+    class Person(Base):
+        __tablename__ = 'people'
+        id = Column(Integer, primary_key=True)
+        name = Column(String(50))
+
+    class Engineer(Person):
+        __tablename__ = 'engineers'
+        __mapper_args__ = {'concrete':True}
+        id = Column(Integer, primary_key=True)
+        primary_language = Column(String(50))
+        name = Column(String(50))
+
+Usage of an abstract base class is a little less straightforward as it
+requires usage of :func:`~sqlalchemy.orm.util.polymorphic_union`,
+which needs to be created with the :class:`.Table` objects
+before the class is built::
+
+    engineers = Table('engineers', Base.metadata,
+                    Column('id', Integer, primary_key=True),
+                    Column('name', String(50)),
+                    Column('primary_language', String(50))
+                )
+    managers = Table('managers', Base.metadata,
+                    Column('id', Integer, primary_key=True),
+                    Column('name', String(50)),
+                    Column('golf_swing', String(50))
+                )
+
+    punion = polymorphic_union({
+        'engineer':engineers,
+        'manager':managers
+    }, 'type', 'punion')
+
+    class Person(Base):
+        __table__ = punion
+        __mapper_args__ = {'polymorphic_on':punion.c.type}
+
+    class Engineer(Person):
+        __table__ = engineers
+        __mapper_args__ = {'polymorphic_identity':'engineer', 'concrete':True}
+
+    class Manager(Person):
+        __table__ = managers
+        __mapper_args__ = {'polymorphic_identity':'manager', 'concrete':True}
+
+.. _declarative_concrete_helpers:
+
+Using the Concrete Helpers
+^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Helper classes provides a simpler pattern for concrete inheritance.
+With these objects, the ``__declare_last__`` helper is used to configure the "polymorphic"
+loader for the mapper after all subclasses have been declared.
+
+.. versionadded:: 0.7.3
+
+An abstract base can be declared using the :class:`.AbstractConcreteBase` class::
+
+    from sqlalchemy.ext.declarative import AbstractConcreteBase
+
+    class Employee(AbstractConcreteBase, Base):
+        pass
+
+To have a concrete ``employee`` table, use :class:`.ConcreteBase` instead::
+
+    from sqlalchemy.ext.declarative import ConcreteBase
+
+    class Employee(ConcreteBase, Base):
+        __tablename__ = 'employee'
+        employee_id = Column(Integer, primary_key=True)
+        name = Column(String(50))
+        __mapper_args__ = {
+                        'polymorphic_identity':'employee',
+                        'concrete':True}
+
+
+Either ``Employee`` base can be used in the normal fashion::
+
+    class Manager(Employee):
+        __tablename__ = 'manager'
+        employee_id = Column(Integer, primary_key=True)
+        name = Column(String(50))
+        manager_data = Column(String(40))
+        __mapper_args__ = {
+                        'polymorphic_identity':'manager',
+                        'concrete':True}
+
+    class Engineer(Employee):
+        __tablename__ = 'engineer'
+        employee_id = Column(Integer, primary_key=True)
+        name = Column(String(50))
+        engineer_info = Column(String(40))
+        __mapper_args__ = {'polymorphic_identity':'engineer',
+                        'concrete':True}
+
+
+.. _declarative_mixins:
+
+Mixin and Custom Base Classes
+==============================
+
+A common need when using :mod:`~sqlalchemy.ext.declarative` is to
+share some functionality, such as a set of common columns, some common
+table options, or other mapped properties, across many
+classes.  The standard Python idioms for this is to have the classes
+inherit from a base which includes these common features.
+
+When using :mod:`~sqlalchemy.ext.declarative`, this idiom is allowed
+via the usage of a custom declarative base class, as well as a "mixin" class
+which is inherited from in addition to the primary base.  Declarative
+includes several helper features to make this work in terms of how
+mappings are declared.   An example of some commonly mixed-in
+idioms is below::
+
+    from sqlalchemy.ext.declarative import declared_attr
+
+    class MyMixin(object):
+
+        @declared_attr
+        def __tablename__(cls):
+            return cls.__name__.lower()
+
+        __table_args__ = {'mysql_engine': 'InnoDB'}
+        __mapper_args__= {'always_refresh': True}
+
+        id =  Column(Integer, primary_key=True)
+
+    class MyModel(MyMixin, Base):
+        name = Column(String(1000))
+
+Where above, the class ``MyModel`` will contain an "id" column
+as the primary key, a ``__tablename__`` attribute that derives
+from the name of the class itself, as well as ``__table_args__``
+and ``__mapper_args__`` defined by the ``MyMixin`` mixin class.
+
+There's no fixed convention over whether ``MyMixin`` precedes
+``Base`` or not.  Normal Python method resolution rules apply, and
+the above example would work just as well with::
+
+    class MyModel(Base, MyMixin):
+        name = Column(String(1000))
+
+This works because ``Base`` here doesn't define any of the
+variables that ``MyMixin`` defines, i.e. ``__tablename__``,
+``__table_args__``, ``id``, etc.   If the ``Base`` did define
+an attribute of the same name, the class placed first in the
+inherits list would determine which attribute is used on the
+newly defined class.
+
+Augmenting the Base
+~~~~~~~~~~~~~~~~~~~
+
+In addition to using a pure mixin, most of the techniques in this
+section can also be applied to the base class itself, for patterns that
+should apply to all classes derived from a particular base.  This
+is achieved using the ``cls`` argument of the :func:`.declarative_base` function::
+
+    from sqlalchemy.ext.declarative import declared_attr
+
+    class Base(object):
+        @declared_attr
+        def __tablename__(cls):
+            return cls.__name__.lower()
+
+        __table_args__ = {'mysql_engine': 'InnoDB'}
+
+        id =  Column(Integer, primary_key=True)
+
+    from sqlalchemy.ext.declarative import declarative_base
+
+    Base = declarative_base(cls=Base)
+
+    class MyModel(Base):
+        name = Column(String(1000))
+
+Where above, ``MyModel`` and all other classes that derive from ``Base`` will have
+a table name derived from the class name, an ``id`` primary key column, as well as
+the "InnoDB" engine for MySQL.
+
+Mixing in Columns
+~~~~~~~~~~~~~~~~~
+
+The most basic way to specify a column on a mixin is by simple
+declaration::
+
+    class TimestampMixin(object):
+        created_at = Column(DateTime, default=func.now())
+
+    class MyModel(TimestampMixin, Base):
+        __tablename__ = 'test'
+
+        id =  Column(Integer, primary_key=True)
+        name = Column(String(1000))
+
+Where above, all declarative classes that include ``TimestampMixin``
+will also have a column ``created_at`` that applies a timestamp to
+all row insertions.
+
+Those familiar with the SQLAlchemy expression language know that
+the object identity of clause elements defines their role in a schema.
+Two ``Table`` objects ``a`` and ``b`` may both have a column called
+``id``, but the way these are differentiated is that ``a.c.id``
+and ``b.c.id`` are two distinct Python objects, referencing their
+parent tables ``a`` and ``b`` respectively.
+
+In the case of the mixin column, it seems that only one
+:class:`.Column` object is explicitly created, yet the ultimate
+``created_at`` column above must exist as a distinct Python object
+for each separate destination class.  To accomplish this, the declarative
+extension creates a **copy** of each :class:`.Column` object encountered on
+a class that is detected as a mixin.
+
+This copy mechanism is limited to simple columns that have no foreign
+keys, as a :class:`.ForeignKey` itself contains references to columns
+which can't be properly recreated at this level.  For columns that
+have foreign keys, as well as for the variety of mapper-level constructs
+that require destination-explicit context, the
+:func:`~.declared_attr` decorator is provided so that
+patterns common to many classes can be defined as callables::
+
+    from sqlalchemy.ext.declarative import declared_attr
+
+    class ReferenceAddressMixin(object):
+        @declared_attr
+        def address_id(cls):
+            return Column(Integer, ForeignKey('address.id'))
+
+    class User(ReferenceAddressMixin, Base):
+        __tablename__ = 'user'
+        id = Column(Integer, primary_key=True)
+
+Where above, the ``address_id`` class-level callable is executed at the
+point at which the ``User`` class is constructed, and the declarative
+extension can use the resulting :class:`.Column` object as returned by
+the method without the need to copy it.
+
+.. versionchanged:: > 0.6.5
+    Rename 0.6.5 ``sqlalchemy.util.classproperty`` into :func:`~.declared_attr`.
+
+Columns generated by :func:`~.declared_attr` can also be
+referenced by ``__mapper_args__`` to a limited degree, currently
+by ``polymorphic_on`` and ``version_id_col``, by specifying the
+classdecorator itself into the dictionary - the declarative extension
+will resolve them at class construction time::
+
+    class MyMixin:
+        @declared_attr
+        def type_(cls):
+            return Column(String(50))
+
+        __mapper_args__= {'polymorphic_on':type_}
+
+    class MyModel(MyMixin, Base):
+        __tablename__='test'
+        id =  Column(Integer, primary_key=True)
+
+Mixing in Relationships
+~~~~~~~~~~~~~~~~~~~~~~~
+
+Relationships created by :func:`~sqlalchemy.orm.relationship` are provided
+with declarative mixin classes exclusively using the
+:func:`.declared_attr` approach, eliminating any ambiguity
+which could arise when copying a relationship and its possibly column-bound
+contents. Below is an example which combines a foreign key column and a
+relationship so that two classes ``Foo`` and ``Bar`` can both be configured to
+reference a common target class via many-to-one::
+
+    class RefTargetMixin(object):
+        @declared_attr
+        def target_id(cls):
+            return Column('target_id', ForeignKey('target.id'))
+
+        @declared_attr
+        def target(cls):
+            return relationship("Target")
+
+    class Foo(RefTargetMixin, Base):
+        __tablename__ = 'foo'
+        id = Column(Integer, primary_key=True)
+
+    class Bar(RefTargetMixin, Base):
+        __tablename__ = 'bar'
+        id = Column(Integer, primary_key=True)
+
+    class Target(Base):
+        __tablename__ = 'target'
+        id = Column(Integer, primary_key=True)
+
+:func:`~sqlalchemy.orm.relationship` definitions which require explicit
+primaryjoin, order_by etc. expressions should use the string forms
+for these arguments, so that they are evaluated as late as possible.
+To reference the mixin class in these expressions, use the given ``cls``
+to get it's name::
+
+    class RefTargetMixin(object):
+        @declared_attr
+        def target_id(cls):
+            return Column('target_id', ForeignKey('target.id'))
+
+        @declared_attr
+        def target(cls):
+            return relationship("Target",
+                primaryjoin="Target.id==%s.target_id" % cls.__name__
+            )
+
+Mixing in deferred(), column_property(), etc.
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+Like :func:`~sqlalchemy.orm.relationship`, all
+:class:`~sqlalchemy.orm.interfaces.MapperProperty` subclasses such as
+:func:`~sqlalchemy.orm.deferred`, :func:`~sqlalchemy.orm.column_property`,
+etc. ultimately involve references to columns, and therefore, when
+used with declarative mixins, have the :func:`.declared_attr`
+requirement so that no reliance on copying is needed::
+
+    class SomethingMixin(object):
+
+        @declared_attr
+        def dprop(cls):
+            return deferred(Column(Integer))
+
+    class Something(SomethingMixin, Base):
+        __tablename__ = "something"
+
+
+Controlling table inheritance with mixins
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+The ``__tablename__`` attribute in conjunction with the hierarchy of
+classes involved in a declarative mixin scenario controls what type of
+table inheritance, if any,
+is configured by the declarative extension.
+
+If the ``__tablename__`` is computed by a mixin, you may need to
+control which classes get the computed attribute in order to get the
+type of table inheritance you require.
+
+For example, if you had a mixin that computes ``__tablename__`` but
+where you wanted to use that mixin in a single table inheritance
+hierarchy, you can explicitly specify ``__tablename__`` as ``None`` to
+indicate that the class should not have a table mapped::
+
+    from sqlalchemy.ext.declarative import declared_attr
+
+    class Tablename:
+        @declared_attr
+        def __tablename__(cls):
+            return cls.__name__.lower()
+
+    class Person(Tablename, Base):
+        id = Column(Integer, primary_key=True)
+        discriminator = Column('type', String(50))
+        __mapper_args__ = {'polymorphic_on': discriminator}
+
+    class Engineer(Person):
+        __tablename__ = None
+        __mapper_args__ = {'polymorphic_identity': 'engineer'}
+        primary_language = Column(String(50))
+
+Alternatively, you can make the mixin intelligent enough to only
+return a ``__tablename__`` in the event that no table is already
+mapped in the inheritance hierarchy. To help with this, a
+:func:`~sqlalchemy.ext.declarative.has_inherited_table` helper
+function is provided that returns ``True`` if a parent class already
+has a mapped table.
+
+As an example, here's a mixin that will only allow single table
+inheritance::
+
+    from sqlalchemy.ext.declarative import declared_attr
+    from sqlalchemy.ext.declarative import has_inherited_table
+
+    class Tablename(object):
+        @declared_attr
+        def __tablename__(cls):
+            if has_inherited_table(cls):
+                return None
+            return cls.__name__.lower()
+
+    class Person(Tablename, Base):
+        id = Column(Integer, primary_key=True)
+        discriminator = Column('type', String(50))
+        __mapper_args__ = {'polymorphic_on': discriminator}
+
+    class Engineer(Person):
+        primary_language = Column(String(50))
+        __mapper_args__ = {'polymorphic_identity': 'engineer'}
+
+If you want to use a similar pattern with a mix of single and joined
+table inheritance, you would need a slightly different mixin and use
+it on any joined table child classes in addition to their parent
+classes::
+
+    from sqlalchemy.ext.declarative import declared_attr
+    from sqlalchemy.ext.declarative import has_inherited_table
+
+    class Tablename(object):
+        @declared_attr
+        def __tablename__(cls):
+            if (has_inherited_table(cls) and
+                Tablename not in cls.__bases__):
+                return None
+            return cls.__name__.lower()
+
+    class Person(Tablename, Base):
+        id = Column(Integer, primary_key=True)
+        discriminator = Column('type', String(50))
+        __mapper_args__ = {'polymorphic_on': discriminator}
+
+    # This is single table inheritance
+    class Engineer(Person):
+        primary_language = Column(String(50))
+        __mapper_args__ = {'polymorphic_identity': 'engineer'}
+
+    # This is joined table inheritance
+    class Manager(Tablename, Person):
+        id = Column(Integer, ForeignKey('person.id'), primary_key=True)
+        preferred_recreation = Column(String(50))
+        __mapper_args__ = {'polymorphic_identity': 'engineer'}
+
+Combining Table/Mapper Arguments from Multiple Mixins
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+In the case of ``__table_args__`` or ``__mapper_args__``
+specified with declarative mixins, you may want to combine
+some parameters from several mixins with those you wish to
+define on the class iteself. The
+:func:`.declared_attr` decorator can be used
+here to create user-defined collation routines that pull
+from multiple collections::
+
+    from sqlalchemy.ext.declarative import declared_attr
+
+    class MySQLSettings(object):
+        __table_args__ = {'mysql_engine':'InnoDB'}
+
+    class MyOtherMixin(object):
+        __table_args__ = {'info':'foo'}
+
+    class MyModel(MySQLSettings, MyOtherMixin, Base):
+        __tablename__='my_model'
+
+        @declared_attr
+        def __table_args__(cls):
+            args = dict()
+            args.update(MySQLSettings.__table_args__)
+            args.update(MyOtherMixin.__table_args__)
+            return args
+
+        id =  Column(Integer, primary_key=True)
+
+Creating Indexes with Mixins
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To define a named, potentially multicolumn :class:`.Index` that applies to all
+tables derived from a mixin, use the "inline" form of :class:`.Index` and establish
+it as part of ``__table_args__``::
+
+    class MyMixin(object):
+        a =  Column(Integer)
+        b =  Column(Integer)
+
+        @declared_attr
+        def __table_args__(cls):
+            return (Index('test_idx_%s' % cls.__tablename__, 'a', 'b'),)
+
+    class MyModel(MyMixin, Base):
+        __tablename__ = 'atable'
+        c =  Column(Integer,primary_key=True)
+
+Special Directives
+==================
+
+``__declare_last__()``
+~~~~~~~~~~~~~~~~~~~~~~
+
+The ``__declare_last__()`` hook allows definition of
+a class level function that is automatically called by the :meth:`.MapperEvents.after_configured`
+event, which occurs after mappings are assumed to be completed and the 'configure' step
+has finished::
+
+    class MyClass(Base):
+        @classmethod
+        def __declare_last__(cls):
+            ""
+            # do something with mappings
+
+.. versionadded:: 0.7.3
+
+.. _declarative_abstract:
+
+``__abstract__``
+~~~~~~~~~~~~~~~~~~~
+
+``__abstract__`` causes declarative to skip the production
+of a table or mapper for the class entirely.  A class can be added within a hierarchy
+in the same way as mixin (see :ref:`declarative_mixins`), allowing subclasses to extend
+just from the special class::
+
+    class SomeAbstractBase(Base):
+        __abstract__ = True
+
+        def some_helpful_method(self):
+            ""
+
+        @declared_attr
+        def __mapper_args__(cls):
+            return {"helpful mapper arguments":True}
+
+    class MyMappedClass(SomeAbstractBase):
+        ""
+
+One possible use of ``__abstract__`` is to use a distinct :class:`.MetaData` for different
+bases::
+
+    Base = declarative_base()
+
+    class DefaultBase(Base):
+        __abstract__ = True
+        metadata = MetaData()
+
+    class OtherBase(Base):
+        __abstract__ = True
+        metadata = MetaData()
+
+Above, classes which inherit from ``DefaultBase`` will use one :class:`.MetaData` as the
+registry of tables, and those which inherit from ``OtherBase`` will use a different one.
+The tables themselves can then be created perhaps within distinct databases::
+
+    DefaultBase.metadata.create_all(some_engine)
+    OtherBase.metadata_create_all(some_other_engine)
+
+.. versionadded:: 0.7.3
+
+Class Constructor
+=================
+
+As a convenience feature, the :func:`declarative_base` sets a default
+constructor on classes which takes keyword arguments, and assigns them
+to the named attributes::
+
+    e = Engineer(primary_language='python')
+
+Sessions
+========
+
+Note that ``declarative`` does nothing special with sessions, and is
+only intended as an easier way to configure mappers and
+:class:`~sqlalchemy.schema.Table` objects.  A typical application
+setup using :func:`~sqlalchemy.orm.scoped_session` might look like::
+
+    engine = create_engine('postgresql://scott:tiger@localhost/test')
+    Session = scoped_session(sessionmaker(autocommit=False,
+                                          autoflush=False,
+                                          bind=engine))
+    Base = declarative_base()
+
+Mapped instances then make usage of
+:class:`~sqlalchemy.orm.session.Session` in the usual way.
+
+"""
+
+from .api import declarative_base, synonym_for, comparable_using, \
+    instrument_declarative, ConcreteBase, AbstractConcreteBase, \
+    DeclarativeMeta, DeferredReflection, has_inherited_table,\
+    declared_attr
+
+
+__all__ = ['declarative_base', 'synonym_for', 'has_inherited_table',
+            'comparable_using', 'instrument_declarative', 'declared_attr',
+            'ConcreteBase', 'AbstractConcreteBase', 'DeclarativeMeta',
+            'DeferredReflection']