version id programmatically outside of the generator.   using this system,
we can also leave the version id alone.

to rely upon server generated version identifiers, using triggers
or other database-provided versioning features, by passing the value
``False``.  The ORM will use RETURNING when available to immediately
load the new version identifier, else it will emit a second SELECT.
[ticket:2793]
- The ``eager_defaults`` flag of :class:`.Mapper` will now allow the
newly generated default values to be fetched using an inline
RETURNING clause, rather than a second SELECT statement, for backends
that support RETURNING.
- Added a new variant to :meth:`.ValuesBase.returning` called
:meth:`.ValuesBase.return_defaults`; this allows arbitrary columns
to be added to the RETURNING clause of the statement without interfering
with the compilers usual "implicit returning" feature, which is used to
efficiently fetch newly generated primary key values.  For supporting
backends, a dictionary of all fetched values is present at
:attr:`.ResultProxy.returned_defaults`.
- add a glossary entry for RETURNING
- add documentation for version id generation, [ticket:867]

- rework the event system so that event modules load after their
targets, dependencies are reversed
- create an improved strategy lookup system for the ORM
- rework the ORM to have very few import cycles
- move out "importlater" to just util.dependency
- other tricks to cross-populate modules in as clear a way as possible

the import structure of many core modules.
``sqlalchemy.schema`` and ``sqlalchemy.types``
remain in the top-level package, but are now just lists of names
that pull from within ``sqlalchemy.sql``.  Their implementations
are now broken out among ``sqlalchemy.sql.type_api``, ``sqlalchemy.sql.sqltypes``,
``sqlalchemy.sql.schema`` and ``sqlalchemy.sql.ddl``, the last of which was
moved from ``sqlalchemy.engine``.  ``sqlalchemy.sql.expression`` is also
a namespace now which pulls implementations mostly from ``sqlalchemy.sql.elements``,
``sqlalchemy.sql.selectable``, and ``sqlalchemy.sql.dml``.
Most of the "factory" functions
used to create SQL expression objects have been moved to classmethods
or constructors, which are exposed in ``sqlalchemy.sql.expression``
using a programmatic system.  Care has been taken such that all the
original import namespaces remain intact and there should be no impact
on any existing applications.   The rationale here was to break out these
very large modules into smaller ones, provide more manageable lists
of function names, to greatly reduce "import cycles" and clarify the
up-front importing of names, and to remove the need for redundant
functions and documentation throughout the expression package.

value is present

mapped class where the polymorphic discriminator has been assigned
to a value that is invalid for the class.   [ticket:2750]

you're dealing with aliased() or with_polymorphic() you need to say "flat=True".  Just the one
flag though, "flat" implies "aliased".