This is an old parameter no longer relevant to how SQLAlchemy
works, once the Query object was introduced.  By deprecating it
we establish that we aren't supporting non-working use cases
and that we encourage applications to move off of the use of this
parameter.

Fixes: #3394
Change-Id: I25b9a38142a1537bbcb27d3e8b66a8b265140072

available mapper options.   This allows a DELETE to proceed
for a joined-table inheritance mapping against the base table only,
while allowing for ON DELETE CASCADE to handle deleting the row
from the subclass tables.
fixes #2349

a base class and a concrete-inherited subclass would raise an error
if those relationships were set up using "backref", while setting up the
identical configuration using relationship() instead with the conflicting
names would succeed, as is allowed in the case of a concrete mapping.
fixes #3630

insert-default holding columns not otherwise included in the SET
clause (such as primary key cols) to get rendered into the RETURNING
even though this is an UPDATE.

- Major fixes to the :paramref:`.Mapper.eager_defaults` flag, this
flag would not be honored correctly in the case that multiple
UPDATE statements were to be emitted, either as part of a flush
or a bulk update operation.  Additionally, RETURNING
would be emitted unnecessarily within update statements.

fixes #3609

which indicates to the ORM that a positive set of None should be
persisted as the value NULL, instead of omitting the column from
the INSERT statement.  This feature is used both as part of the
implementation for :ticket:`3514` as well as a standalone feature
available on any type.  fixes #3250
- add new documentation section illustrating the "how to force null"
 use case of #3250
- alter our change from #3514 so that the class-level flag is now
called "should_evaluate_none"; so that "evaluates_none" is now
a generative method.

to use mapper.cascade_iterator() for this purpose as it was not really
designed for that use case.  Add docs to cascade_iterator() pointing
to the recipe.  fixes #3498

mapper_configured(), after_configured(), and before_configured().

- Fixes to the ORM and to the postgresql JSON type regarding the
``None`` constant in conjunction with the Postgresql :class:`.JSON` type.  When
the :paramref:`.JSON.none_as_null` flag is left at its default
value of ``False``, the ORM will now correctly insert the Json
"'null'" string into the column whenever the value on the ORM
object is set to the value ``None`` or when the value ``None``
is used with :meth:`.Session.bulk_insert_mappings`,
**including** if the column has a default or server default on it.  This
makes use of a new type-level flag "evaluates_none" which is implemented
by the JSON type based on the none_as_null flag. fixes #3514
- Added a new constant :attr:`.postgresql.JSON.NULL`, indicating
that the JSON NULL value should be used for a value
regardless of other settings. part of fixes #3514

if the name of the attribute being accessed is non-specific,
thereby avoiding collisions on names like items, values, keys.
fixes #3475