Support %(app_label)s and %(class)s in related_name

[thenewguy]

I am trying to add TaggableManager to an Abstract model. I've added a related name like related_name='%(app_label)s_%(class)s_tags'. This gives an error like The name '%(app_label)s_%(class)s_tags' is invalid related_name for field with HINT: Related name must be a valid Python identifier or end with a '+'

Per the Django docs, this is a valid form of related_name.

Shouldn't TaggableManager be performing the string format operation? At a glance this seems like it would work just fine with generic object tagging

[rtpg]

OK so I narrowed this down to the fact that TaggableManager is using a totally custom contribute_to_class. In Django's RelatedField.contribute_to_class method we have

            if related_name:
                related_name = related_name % {
                    'class': cls.__name__.lower(),
                    'model_name': cls._meta.model_name.lower(),
                    'app_label': cls._meta.app_label.lower()
                }
                self.remote_field.related_name = related_name

Ideally I would like for us to just find a way to use contribute_to_class directly, but even a patch that would just copy over this behavior could be pulled into the project.

[rtpg]

Labelling a bug since we are a RelatedField sublcass, so should behave as such when possible.

[thenewguy]

Haven't used this past an initial investigation but it does seem to be the solution:

class AbstractTaggableManager(TaggableManager):
    def contribute_to_class(self, cls, name):
        self.opts = cls._meta

        if not cls._meta.abstract:
            if self.remote_field.related_name:
                related_name = self.remote_field.related_name
            else:
                related_name = self.opts.default_related_name
            if related_name:
                related_name = related_name % {
                    "class": cls.__name__.lower(),
                    "model_name": cls._meta.model_name.lower(),
                    "app_label": cls._meta.app_label.lower(),
                }
                self.remote_field.related_name = related_name
        
        super().contribute_to_class(cls, name)

[thenewguy]

As far as testing goes - would it be good enough to simply define an abstract and concrete implementation in the models? System checks seem to throw an error with a related name like related_name="%(app_label)s_%(model_name)s_%(class)s_tags" without this quickfix

[rtpg]

@thenewguy if you want to take this on, I think the straightforward way would be to just edit one of the existing test models with a related name like what you are discussing, then we can add a quick test/modify an existing one to check that the related name accessor works.

For example one of the models in this file, and editing a test.

And of course for actually getting the bug fixed, just reworking the TaggableManager contribute_to_class function!

Would you be interested in making a PR with your changes? Even just having that as a starting point would be helpful. And if you're not interested, I can look at making a PR instead. But I think you understand the problem well