remove get_by_natural_key implementation on UserProfileManager

[Kakadus]

leftover from #2249.

You can see an effect, if using the "natural key notation" [null]" for a UserProfile fk:

WITHOUT remove get_by_natural_key implementation on UserProfileManager, loading the fixture below results in

django.core.serializers.base.DeserializationError: Problem installing fixture '/home/jonas/development/evap/breakage.json': get() returned more than one UserProfile -- it returned 2!: (evaluation.evaluation:pk=356000) field_value was '[None]'

while WITH the commit, the error is (correctly)

django.core.serializers.base.DeserializationError: Problem installing fixture '/home/jonas/development/evap/breakage.json': ['“[None]” value must be an integer.']: (evaluation.evaluation:pk=356000) field_value was '[None]'

json fixture to load:

[{"model": "evaluation.evaluation","pk": 356000,"fields": {"state": 80,"course": 141,"name_de": "","name_en": "","weight": 1,"is_single_result": false,"is_rewarded": true,"is_midterm_evaluation": false,"can_publish_text_results": true,"_participant_count": 62,"_voter_count": 35,"vote_start_datetime": "2022-02-02T00:00:00","vote_end_date": "2022-02-12","allow_editors_to_edit": true,"wait_for_grade_upload_before_publishing": true,"participants": [[null]],"voters": []}}]

I cannot think of any test for this, but browsing the django code revealed no critical changes.

[Kakadus]

I do want to note here that get_by_natural_keys is required by django.contrib.auth.handers.modwsgi, but we don't use it.

[Kakadus]

Dö döööm. create_super_user tries to check if there is a get_by_natural_key method by checking if the email field is unique. Ours is unique but also null=True

[Kakadus]

Here is the code in question:

    def _validate_username(self, username, verbose_field_name, database):
        """Validate username. If invalid, return a string error message."""
        if self.username_is_unique:
            try:
                self.UserModel._default_manager.db_manager(database).get_by_natural_key(
                    username
                )
            except self.UserModel.DoesNotExist:
                pass
            else: ...

which crashes when we do not have get_by_natural_key implemented. username_is_unique is true because of

    @cached_property
    def username_is_unique(self):
        if self.username_field.unique:
            return True
        return any(
            len(unique_constraint.fields) == 1
            and unique_constraint.fields[0] == self.username_field.name
            for unique_constraint in self.UserModel._meta.total_unique_constraints
        )

and we do have unique=True but also null=False.

https://github.com/django/django/blob/main/django/contrib/auth/management/commands/createsuperuser.py#L291-L309

@niklasmohrin and I concluded that this is a django bug, I will open another ticket later.