- 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

``__eq__()`` to return a non-boolean-capable object, such as
some geoalchemy types as well as numpy types, were being tested
for ``bool()`` during a unit of work update operation, where in
0.9 the return value of ``__eq__()`` was tested against "is True"
to guard against this.
fixes #3469

feature would cause an object's version counter to be incremented
when there was no net change to the object's row, but instead an object
related to it via relationship (e.g. typically many-to-one)
were associated or de-associated with it, resulting in an UPDATE
statement that updates the object's version counter and nothing else.
In the use case where the relatively recent "server side" and/or
"programmatic/conditional" version counter feature were used
(e.g. setting version_id_generator to False), the bug could cause an
UPDATE without a valid SET clause to be emitted.
fixes #3465

to suit #3451 exclude these columns from the "params"
dictionary in the first place, revises pr github:181

Suppose you have a model class with a primary key.

Base = declarative_base()
class User(Base):
  id = Column(BigInteger, primary_key=True)
  name = Column(String)

Previously, running
`bulk_update_mappings(User, {'id': 1, 'name': 'hello'})`
would emit the following:

```UPDATE users SET id=1, name='hello' WHERE id=1```

This is contrary to the stated behaviour, where primary keys are omitted
from the SET clause. Furthermore, this behaviour is harmful, as it
can cause the db engine to lock over-aggresively (at least in Postgres).

With this change, the emitted SQL is:

```UPDATE users SET name='hello' WHERE id=1```

as a result of the bugfix for :ticket:`3167`,
where attribute and validation events are no longer
called within the flush process.  The mutable
extension was relying upon this behavior in the case where a column
level Python-side default were responsible for generating the new value
on INSERT or UPDATE, or when a value were fetched from the RETURNING
clause for "eager defaults" mode.  The new value would not be subject
to any event when populated and the mutable extension could not
establish proper coercion or history listening.  A new event
:meth:`.InstanceEvents.refresh_flush` is added which the mutable
extension now makes use of for this use case.
fixes #3427
- Added new event :meth:`.InstanceEvents.refresh_flush`, invoked
when an INSERT or UPDATE level default value fetched via RETURNING
or Python-side default is invoked within the flush process.  This
is to provide a hook that is no longer present as a result of
:ticket:`3167`, where attribute and validation events are no longer
called within the flush process.
- Added a new semi-public method to :class:`.MutableBase`
:meth:`.MutableBase._get_listen_keys`.  Overriding this method
is needed in the case where a :class:`.MutableBase` subclass needs
events to propagate for attribute keys other than the key to which
the mutable type is associated with, when intercepting the
:meth:`.InstanceEvents.refresh` or
:meth:`.InstanceEvents.refresh_flush` events.  The current example of
this is composites using :class:`.MutableComposite`.

where the check for query state on :meth:`.Query.update` or
:meth:`.Query.delete` compared the empty tuple to itself using ``is``,
which fails on Pypy to produce ``True`` in this case; this would
erronously emit a warning in 0.9 and raise an exception in 1.0.
fixes #3405

set to a SQL expression for an UPDATE, and the SQL expression when
compared to the previous value of the attribute would produce a SQL
comparison other than ``==`` or ``!=``, the exception "Boolean value
of this clause is not defined" would raise.   The fix ensures that
the unit of work will not interpret the SQL expression in this way.
fixes #3402

to transient objects with attributes unset would leak NEVER_SET,
and negated_contains_or_equals would do so for any transient
object as the comparison used only the committed value.
Repaired the NEVER_SET cases, fixes #3371, and also made
negated_contains_or_equals() use state_attr_by_column() just
like a non-negated comparison, fixes #3374