Thanks epandurski at gmail.com for the report.

Thanks @timgraham and @jdunck for the code reviews and Kronuz for bug
report and initial patch.

Schema alteration

Conflicts:
	django/db/backends/oracle/base.py
	django/db/backends/postgresql_psycopg2/base.py
	django/db/models/signals.py
	tests/queries/tests.py

Added settings.SESSION_SERIALIZER which is the import path of a serializer
to use for sessions.

Thanks apollo13, carljm, shaib, akaariai, charettes, and dstufft for reviews.

The view tried to display links to a ModelAdmin's change_view, which
resulted in NoReverseMatches if get_urls was overridden to remove the
corresponding url.

The __eq__ method now considers two instances without primary key value
equal only when they have same id(). The __hash__ method raises
TypeError for no primary key case.

Fixed #18864, fixed #18250

Thanks to Tim Graham for docs review.

This reverts commit ce0e86cf.

The check is necessary if 'ENGINE' is an empty string.
Thanks apollo13 for pointing this out.

In cases where the same connection (from model A to model B along the
same field) was needed multiple times in a select_related query, the
join setup code mistakenly reused an existing join.

Documentation -- Corrected error about Model.full_clean()

refs #20429

Thanks Simon Charette for the report and review.

Thanks simonb for the report, Claude Paroz and Susan Tan for their work
on a fix.

Cleaned up the internal implementation of m2m fields by removing
related.py _get_fk_val(). The _get_fk_val() was doing the wrong thing
if asked for the foreign key value on foreign key to parent model's
primary key when child model had different primary key field.

Added OneToOneField to the list of model fields for which the unique
argument isn't valid. (OneToOneFields are inherently unique, and if
the user supplies a value for unique it is ignored / overwritten.)

Thanks Michael Farrell for the report and his work on the fix.

Tests by Tim Graham, report from jeroen.pulles@redslider.net.

If LEFT JOINs are required for correct results, then trimming the join
can lead to incorrect results. Consider case:

TBL A: ID | TBL B: ID  A_ID
       1           1   1
       2
Now A.order_by('b__a') did use a join to B, and B's a_id column. This
was seen to contain the same value as A's id, and so the join was
trimmed. But this wasn't correct as the join is LEFT JOIN, and for row
A.id = 2 the B.a_id column is NULL.

There was an old complaint about nullable one-to-one field cascading
even when the o2o field was saved to None value before the deletion.
Added an test to verify this doesn't happen.

Also some PEP 8 cleanup.