Migration autodetector changes related_name for self-referential ManyToManyField.

[Mariusz Felisiak]

Migration autodetector no longer adds a model name to the related_name attribute for self-referential ManyToManyField, e.g. for a field

class MyModel2(models.Model):
    field_3 = models.ManyToManyField('self')

it creates a migration with related_name='field_3_rel_+' instead of related_name='_mymodel2_field_3_+'.

Regression in aa4acc164d1247c0de515c959f7b09648b57dc42 (see #29899).

[David Wobrock]

Hi,

Similarly to #32675, on a first and very quick look, the ManyToManyField has some logic at django.db.models.fields.related.ManyToManyField.contribute_to_class that looks very much like the names described in the ticket itself:

    def contribute_to_class(self, cls, name, **kwargs):
        # To support multiple relations to self, it's useful to have a non-None
        # related name on symmetrical relations for internal reasons. The
        # concept doesn't make a lot of sense externally ("you want me to
        # specify *what* on my non-reversible relation?!"), so we set it up
        # automatically. The funky name reduces the chance of an accidental
        # clash.
        if self.remote_field.symmetrical and (
            self.remote_field.model == RECURSIVE_RELATIONSHIP_CONSTANT or
            self.remote_field.model == cls._meta.object_name
        ):
            self.remote_field.related_name = "%s_rel_+" % name
        elif self.remote_field.is_hidden():
            # If the backwards relation is disabled, replace the original
            # related_name with one generated from the m2m field name. Django
            # still uses backwards relations internally and we need to avoid
            # clashes between multiple m2m fields with related_name == '+'.
            self.remote_field.related_name = '_%s_%s_%s_+' % (
                cls._meta.app_label,
                cls.__name__.lower(),
                name,
            )

Since _meta should not be available anymore, I expect that the result of the self.remote_field.model == cls._meta.object_name condition changed, because we should not be able to use cls._meta.app_label.

I'm open to help if we have an idea about how we can fix this :)

[Simon Charette]

That seems like a flaw of RelatedField.deconstruct tbh, it should not include related_name and friends if it was not specified at initialization.

I suggest we make RelatedField.__init__ store related_name and related_query_name as self._related_name and self._related_query_name and use them instead of relying on self.remote_field in deconstruct. That's the approach we've taken for post-initialization alterable attributes in Field ​for example.

[David Wobrock]

Started working on a patch ​https://github.com/django/django/pull/14324
but still Work In Progress

[David Wobrock]

Some discussions occurred on the Pull Request: ​https://github.com/django/django/pull/14324#issuecomment-843038110

Added tests and updated release note, patch is ready for review :) Thanks!

[Mariusz Felisiak <felisiak.mariusz@…>]

In b9df2b7:

Fixed #32676 -- Prevented migrations from rendering related field attributes when not passed during initialization.

Thanks Simon Charette for the implementation idea.