Support callable values in through_defaults.

[Baptiste Mispelon]

Ticket #9475 gave us through_defaults but unlike the defaults argument of get_or_create [1] or the default argument of any model field, it doesn't allow callable values.

Callable values are passed through without being evaluated so the exact behavior depends on the fields. With a CharField for example, the repr() of the function will be saved to the database which is most likely not what the user is expecting.

I took a look at the original ticket and pull request but couldn't find a mention of this use-case (ctrl+F for the word "callable") so it seems that it was an oversight rather than a deliberate design decision.

Code-wise, fixing this is actually pretty straightforward and doesn't seem to cause any regression (see attached pull request).

[1] ​https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.get_or_create

[Baptiste Mispelon]

​PR here (with docs and tests)

[Mariusz Felisiak <felisiak.mariusz@…>]

In 6c0341f1:

Refs #31046 -- Added django.db.models.utils.resolve_callables().

[Mariusz Felisiak <felisiak.mariusz@…>]

In 26cab4e8:

Fixed #31046 -- Allowed RelatedManager.add()/create()/set() to accept callable values in through_defaults.