ForeignObject checks are too strict

[Antoine Catton]

Let's consider this test case:

class Parent(models.Model):
  a = models.Field()
  b = models.Field()

  class Meta:
    unique_together = (
      ('a', 'b'),
    )

class Child(models.Model):
  a = models.Field()
  b = models.Field()
  parent = ForeignObject(Parent, from_fields=('a', 'b'), to_fields=('a', 'b'))

I get the error: None of the fields 'a', 'b' on 'Parent' have a unique=True constraint. But a and b are unique together, the ForeignObject will be functional.

[Antoine Catton]

Here's my patch: ​https://github.com/django/django/pull/5419

[Simon Charette]

I think this is more of a bug than a new feature. The provided patch looks good to me as I'm not convinced this is worth a mention in the release notes.

[Anssi Kääriäinen]

ForeignObject and multicolumn foreign keys using it are private API. So, as long as we don't have changes to public API not release notes is OK for me.

[Simon Charette <charette.s@…>]

In 80dac8c3:

Fixed #25535 -- Made ForeignObject checks less strict.

Check that the foreign object from_fields are a subset of any unique
constraints on the foreign model.

[Tim Graham <timograham@…>]

In c7aff313:

Refs #25535 -- Minor edits to ForeignObject check changes.

[Tim Graham <timograham@…>]

In 38d6e1e2:

[1.9.x] Fixed #25535 -- Made ForeignObject checks less strict.

Check that the foreign object from_fields are a subset of any unique
constraints on the foreign model.

Backport of 80dac8c33e7f6f22577e4346f44e4c5ee89b648c and
c7aff31397a7228f6ac2e33c10ebdf36c4b7a9b7 from master