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 a :class:`.types.Enum` or :class:`.postgresql.ENUM` subtype
will now emit the expected "CREATE TYPE" and "DROP TYPE" DDL when
the type is used within a "CREATE TABLE" or "DROP TABLE".
fixes #2729

ARRAY type, references #3132

for all assorted PG SQL extensions

- any/all work for Array as well as subqueries, accepted by MySQL
- Postgresql ARRAY now subclasses Array
- fixes #3516

- 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

- The "hashable" flag on special datatypes such as :class:`.postgresql.ARRAY`,
:class:`.postgresql.JSON` and :class:`.postgresql.HSTORE` is now
set to False, which allows these types to be fetchable in ORM
queries that include entities within the row.  fixes #3499
- The Postgresql :class:`.postgresql.ARRAY` type now supports multidimensional
indexed access, e.g. expressions such as ``somecol[5][6]`` without
any need for explicit casts or type coercions, provided
that the :paramref:`.postgresql.ARRAY.dimensions` parameter is set to the
desired number of dimensions. fixes #3487
- The return type for the :class:`.postgresql.JSON` and :class:`.postgresql.JSONB`
when using indexed access has been fixed to work like Postgresql itself,
and returns an expression that itself is of type :class:`.postgresql.JSON`
or :class:`.postgresql.JSONB`.  Previously, the accessor would return
:class:`.NullType` which disallowed subsequent JSON-like operators to be
used. part of fixes #3503
- The :class:`.postgresql.JSON`, :class:`.postgresql.JSONB` and
:class:`.postgresql.HSTORE` datatypes now allow full control over the
return type from an indexed textual access operation, either ``column[someindex].astext``
for a JSON type or ``column[someindex]`` for an HSTORE type,
via the :paramref:`.postgresql.JSON.astext_type` and
:paramref:`.postgresql.HSTORE.text_type` parameters. also part of fixes #3503
- The :attr:`.postgresql.JSON.Comparator.astext` modifier no longer
calls upon :meth:`.ColumnElement.cast` implicitly, as PG's JSON/JSONB
types allow cross-casting between each other as well.  Code that
makes use of :meth:`.ColumnElement.cast` on JSON indexed access,
e.g. ``col[someindex].cast(Integer)``, will need to be changed
to call :attr:`.postgresql.JSON.Comparator.astext` explicitly.  This is
part of the refactor in references #3503 for consistency in operator
use.