Add model Field.rel_db_type() method

[Alexander Schepanovski]

rel_db_type() method will return db_type for ForeignKey / OneToOneField to a field,
if not exists value of db_type() method should be used as it's done now.

It will remove a need for a hack in ForeignKey.db_type and make possible subclasses of AutoField work such as:

class BigAutoField(models.AutoField):
    def __init__(self, verbose_name=None, name=None, external_sequence=False, **kwargs):
        self.external_sequence = external_sequence
        models.AutoField.__init__(self, verbose_name, name, **kwargs)

    def db_type(self, connection):
        # now only for PostgreSQL, should be done more carefully to support other backends
        return 'bigint' if self.external_sequence else 'bigserial'

    def rel_db_type(self, connection):
        return 'bigint'

[Russell Keith-Magee]

I'm afraid I don't see why this is required. The explanation in ticket says it's to avoid a hack, but doesn't describe what that hack is required to solve; the patch doesn't contain any test cases to demonstrate why the feature is required (or that it even works as suggested). I'm also not especially inspired by a patch for a new feature that contains a TODO marker. As Yoda would advise: Do or do not... there is no TODO :-)

If this is a precursor to the NoSQL work, then it should be folded into that work; we're not going to accept and modify code that pre-empts the solution introduced by a SoC project.

[Alexander Schepanovski]

Patch is needed not to avoid a hack, but to make possible custom AutoField subclasses as in example.
I'll provide whole rel_db_type() cleanup patch to follow Yoda.

And no, it's not a precursor for NoSQL.

[Alexander Schepanovski]

rel_db_type for all fields, custom for AutoField, PositiveIntegerField and PositiveSmallIntegerField

[Russell Keith-Magee]

Ok - this makes a little more sense now. It still needs tests and documentation, however.

I also have a sneaking suspicion that this may be linked with #11319.

[Tim Graham]

Docs are still missing in the ​PR.

[Tim Graham <timograham@…>]

In b61eab18:

Fixed #13774 -- Added models.Field.rel_db_type().