Automatically add clean functionality for partially unique index constraints?

[sjamaan]

Django has no validation for unique indexes (see this part of the Django code). As far as I can tell, you can pretty easily query partially unique fields just as easily as you can query fully unique(_together) fields.

For projects we've been using our own hacked version of the django-partial-index plugin, which provides a mixin that adds validations for these kind of partial indexes. We could migrate away from this plugin and use standard Django indexes with conditions, if Binder model would extend full_clean to check partial indexes as well.

Also: Maybe make a PR for Django for this so in the long run we can drop this again?