UniqueConstraint message does not use violation_error_message

[Gerben Morsink]

Although this is documented, it is unexpected. Therefore I filed it as a cleanup/optimization.

See ​https://forum.djangoproject.com/t/customizing-validationerror-message-on-uniqueconstraint/25105 for a (possible) patch.

[Tim Graham]

While Simon did provide a draft patch on the forum, I'll uncheck "Has patch" until there is a pull request to review.

[Gerben Morsink]

I created a patch in ​https://github.com/django/django/pull/17723

[Claude Paroz]

Please do not close a ticket before its patch is committed in the repository. Unsetting the "Needs tests" flag should be enough to promote the patch in the review queue again!

[Gerben Morsink]

Somehow a lot of seemingly unrelated tests are failing on my local machine. It could have to do with https://code.djangoproject.com/ticket/30581 being merged in between the first release and the current release.

@Sarah Boyce: do you know why these lines were changed:
​https://github.com/django/django/commit/13922580cccfb9ab2922ff4943dd39da56dfbd8c ?

[Sarah Boyce <42296566+sarahboyce@…>]

In e970bb7:

Fixed #35103 -- Used provided error code and message when fields is set without a condition on UniqueConstraint.

[Brendan]

I think this is still perhaps not working as intended, in particular when using expressions. We can see with the simple case:

from django.db import models

class MyModel(models.Model)
    name = models.CharField()

    class Meta:
        constraints = (
            models.UniqueConstraint(
                Lower("name"),
                name="unique_supplier_name",
                violation_error_message="test"
            ),
        )

If for some reason it should be obvious and expected that violation_error_message should not apply when using expressions, I think we could perhaps clean up the docs to clarify this (and maybe why).