BaseModelMutation doesn't generate input fields for reverse foreign key relationships.

[gghildyal]

For example, say you have two models (Just psuedocode)

class Father(models.Model):
   name = CharField(max_length=256, db_index=True)

class Son(models.Model):
   name = CharField(max_length=256, db_index=True)
   father = ForeignKey(Father, on_delete=CASCADE, related_name='sons')

And then my update mutation object would look something like this -

class UpdateFather(ModelUpdateMutation):
   class Meta:
      model = Father

For updates, I would like to attach or link sons to father when I update a father.

This is currently not possible because BaseModelMutation calls _get_fields which does not handle model._meta.related_objects and ManyToOne fields. So the schema never gets the reverse ManyToOne field included as a list of IDs in the input argument.

Support of handling reverse relationships is highly useful, it will support UI workflows similar to how the admin UI can support creating and linking child models to parent while updating the parent.

[bellini666]

Fixed at #12