Postgres "ExclusionConstraint with conditions cannot be deferred." seems wrong

[Márton Salomváry]

When attempting to add a constraint to a model that looks something like this:

ExclusionConstraint(
                name="my_constraint",
                expressions=[
                    ("is_default", RangeOperators.EQUAL),
                ],
                condition=Q(is_default=False),
                deferrable=Deferrable.DEFERRED,
            )

makemigrations fails with the following error:

    ExclusionConstraint(
  File "django/contrib/postgres/constraints.py", line 55, in __init__
    raise ValueError("ExclusionConstraint with conditions cannot be deferred.")
ValueError: ExclusionConstraint with conditions cannot be deferred.

However PostgreSQL 14.5 seems to be perfectly fine with the following:

ALTER TABLE my_model
    ADD CONSTRAINT my_constraint
        EXCLUDE (is_default WITH =)
        WHERE (is_default = TRUE)
        INITIALLY DEFERRED;

I've tried to figure out why this restriction may be in place but neither other parts of Django nor PostgreSQL itself seem to require deferred exclusion constraints to not have a condition.

The commit that added this also does not seem to explain the situation: ​https://github.com/django/django/commit/b4068bc65636cca6c2905aa8c40bea69bb0e4245

Maybe an accidental copypaste from here? ​https://github.com/django/django/blob/35911078fa40eb35859832987fedada76963c01e/django/db/models/constraints.py#L153-L154

[Simon Charette]

I think this is effectively a bug due to copy-pasting.

Django implements partial unique constraints using partial unique indices which ​lack support for the constraint specific options.

Exclusion constraint use proper CONSTRAINT syntax so I don't see why they couldn't support deferral specifiers.

Would you be willing to submit a patch to correct the issue?

[Márton Salomváry]

Opened a pull request here: ​https://github.com/django/django/pull/16278

[Mariusz Felisiak <felisiak.mariusz@…>]

In d6cbf39:

Fixed #34149 -- Allowed adding deferrable conditional exclusion constraints on PostgreSQL.

[Simon Charette]

Bhuvnesh, it's great that you are enthusiastic about fixing issues but you might want to give reporters a chance to reply on whether or not they want to provide a patch themselves before assigning the ticket to you.