Extend deferred unique constraint support to OneToOneField

[BorisZZZ]

Support for DEFERRABLE INITIALLY DEFERRED was added in https://code.djangoproject.com/ticket/20581 . It would be great if it could be extended to the implicit Unique constraint generated by OneToOneField, say, by adding an argument:
OneToOneField(OtherModel, defer=models.Deferrable.DEFERRED)

[Simon Charette]

Not sure we should do that as it seems pretty niche.

I guess an alternative API could be to allow passing a UniqueConstraint to the unique kwarg like we've discussed doing to the Field.index one.

e.g.

OneToOneField(OtherModel, unique=UniqueConstraint(defer=models.Deferrable.DEFERRED))

[Petr Přikryl]

I think it would be quite easy. Here we could pass deferrable argument based on new_field.defer value:

​https://github.com/django/django/blob/4.0/django/db/backends/base/schema.py#L810
​https://github.com/django/django/blob/4.0/django/db/backends/base/schema.py#L1245

[jefflieb-wavy]

There is a use case for this, ordering of database rows using a linked list. OneToOne can be used to define the field pointing to the previous row, but the associated constraint needs to be deferred to allow for updates that temporarily violate the constraint while changing the row ordering.

We were able to implement this, but it involved a workaround. Specifically we had to manually delete the constraint created by the OneToOne and then recreate it with deferred in our migrations, then add that constraint to the model definition also.