Deprecate direct assignment to the reverse side of a related set

[Tim Graham]

As ​proposed by Ansii on django-developers:

>>> obj.reverse_fk_set = RelatedObj.objects.all()

What happens is that the RelatedObj.objects.all() queryset is
iterated and each object of the queryset is saved with the fk changed
to point to the obj. An implicit save.

Now, I don't like the implicit saves at all. A variable assignment
should not cause a database save. So, I would like to deprecate the
current behavior. Assignment to reverse_fk_set (and I guess this goes
for m2m, too) is no longer allowed. Instead you will need to
explicitly do:

 >>> obj.reverse_fk_set.set(RelatedObj.objects.all())

Now you are calling a method, and saving in this situation is OK and
analogous to other related manager methods.

Lets raise a deprecation warning on direct assignment to the
reverse_fk_set and remove it. The message would be
something like "Direct assignment to the reverse side of a related set
is deprecated. Use .set() instead."

[Tim Graham]

​PR

[Tim Graham <timograham@…>]

In 9c5e2728:

Fixed #25550 -- Deprecated direct assignment to the reverse side of a related set.

[Tim Graham <timograham@…>]

In c7b8d5d:

Refs #25550 -- Removed reverse related set assignment in selenium tests.

[GitHub <noreply@…>]

In 5fa43705:

Refs #25550 -- Corrected deprecation message for assigning M2M relations.

[Tim Graham <timograham@…>]

In 83b950e9:

[1.10.x] Refs #25550 -- Corrected deprecation message for assigning M2M relations.

Backport of 5fa4370543658aedd79dc554d8c52684d6c7cbca from master

[Tim Graham <timograham@…>]

In 8dac9890:

Refs #25550 -- Removed a deprecated reverse assignment example in docs.

[Tim Graham <timograham@…>]

In e1f93e2b:

[1.10.x] Refs #25550 -- Removed a deprecated reverse assignment example in docs.

Backport of 8dac9890a5941e7b59ea0ebebed400965941fe09 from master

[Tim Graham <timograham@…>]

In ed251246:

Refs #25550 -- Removed support for direct assignment to the reverse side of a related set.