Check that CheckConstraint.check and UniqueConstraint.condition don't span joins.

[Simon Charette]

Similar to #31410 but for check and condition.

---

Not everyone is familiar with the fact database level constraint cannot span across tables and might be tempted to do

class Person(models.Model):
    age = models.PositiveSmallIntegerField()
    parent = models.ForeignKey(self)

    class Meta:
        constraints = {
            CheckConstraint(
                name='age_lt_parent', check=Q(age__lt=parent__age)
            ),
        }

Which we'll happily create migrations for but we'll then crash because we prevent JOINs when resolving check.

[Hasan Ramezani]

​PR

[Alexandr Tatarinov]

Is it even possible to handle all cases with Q objects?
As mentioned in this comment, custom expressions might be impossible to introspect.
Maybe we can rely on something like get_source_expressions to get all included fields...

[Hasan Ramezani]

Here is my ​comment on PR:

Yes, this doesn't work for Lower('parent__name'). I can change the patch to cover this case as well but I don't know exactly which kind of expression should I cover? and with more cases, the code is going to be complicated.

@felixxm Do you have any idea?

[Mariusz Felisiak <felisiak.mariusz@…>]

In b7b7df5:

Fixed #31530 -- Added system checks for invalid model field names in CheckConstraint.check and UniqueConstraint.condition.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 33abc556:

Refs #31530 -- Added test for joined OneToOneField in CheckConstraint.check