RenameModel does not rename M2M column when run after RenameField

[David Nelson Adamec]

Encountered this on a project at work. I have some models like so:

class ModelA(models.Model):
    name_renamed = models.CharField(max_length=10)

class ModelBRenamed(models.Model):
    a = models.ForeignKey(ModelA, null=True)

class ModelC(models.Model):
    b_set = models.ManyToManyField(ModelBRenamed)

I have a migration 0002_renamefield to rename a field on ModelA:

class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0001_initial'),
    ]

    operations = [
        migrations.RenameField(
            model_name='modela',
            old_name='name',
            new_name='name_renamed',
        ),
    ]

Then a migration 0003_renamemodel to rename ModelB:

class Migration(migrations.Migration):

    dependencies = [
        ('myapp', '0002_renamefield'),
    ]

    operations = [
        migrations.RenameModel(
            old_name='ModelB',
            new_name='ModelBRenamed',
        ),
    ]

If the two migrations are run together, then the modelb_id column in myapp_modelc_b_set will not be renamed to modelbrenamed_id. However, if I run the migrations one at a time, the column will be renamed as expected.

I think this is related to #27737. When ModelA is reloaded in the RenameField migration, ModelB is reloaded due to its foreign key but is missing its relationship to ModelC. When the RenameModel migration is run, ModelC is not included in ModelB's related_objects, so the through table's column is not renamed.

I've reproduced this using Django 1.11 on either MySQL or SQLite.

[Tim Graham]

Please test with Django 2.0 as there are changes there which may resolve this.

[David Nelson Adamec]

Sorry, should have just done that from the start. Tested in Django 2.0.1, and the issue still occurs.

[Tim Graham]

Thanks, good bug report. I can also reproduce on PostgreSQL with Django master (e2908ecb3e20a629be801ddb826c474f7e7023ea).

[David Nelson Adamec]

After doing a little more digging, it appears that the reload_model with delay=True in the RenameField migration is the culprit. Because of the delay flag, only the original model and its directly related models (ModelA and ModelB in the example) are reloaded, so ModelC is untouched and its b_set field continues to point at the old version of ModelB that's no longer registered. Since ModelC isn't pointing at the new ModelB, it's not included in ModelB's related_objects.

I'm not sure what the performance implication would be if delayed loading were removed from RenameField/AlterField, but I'm sure it's not good. Perhaps there's another clever solution out there, but I feel a bit out of my depth. Here's the workaround I'm using for the time being.

class RenameModelWorkaround(migrations.RenameModel):
    def database_forwards(self, app_label, schema_editor, from_state, to_state):
        from_state.reload_model(app_label, self.old_name_lower)
        super(RenameModelWorkaround, self).database_forwards(
            app_label, schema_editor, from_state, to_state)

[Jeff]

Unsurprisingly, this bug also breaks backwards migrations, as the names in the bridge table don't match expected values.

For the models/migrations in the tickets description, backwards migrations produces the expected result:

django.db.utils.OperationalError: (1054, "Unknown column 'modelbrenamed_id' in 'temp_modelc_b_set'")

David Nelson Adamec was also correct, removing 'delay=delay' from the state.reload_model() call in state_forwards() did fix this particular issue, but I am unsure as to what downsides are associated with removing the delay. I am looking into other solutions now.

[Jeff]

​PR

[Tim Graham <timograham@…>]

In fcc4e251:

Fixed #29000 -- Fixed RenameModel's renaming of a M2M column when run after RenameField.

Regression in 45ded053b1f4320284aa5dac63052f6d1baefea9.

[Tim Graham <timograham@…>]

In 2b7b1993:

[2.1.x] Fixed #29000 -- Fixed RenameModel's renaming of a M2M column when run after RenameField.

Regression in 45ded053b1f4320284aa5dac63052f6d1baefea9.

Backport of fcc4e251dbc917118f73d7187ee2f4cbf3883f36 from master

[Mariusz Felisiak <felisiak.mariusz@…>]

In ad81133:

Refs #29000 -- Restored delayed model rendering of RenameField.

Non-delayed rendering is unnecessary and wasteful now that state models
relationship consistency on delayed reload is ensured.

This partly reverts commit fcc4e251dbc917118f73d7187ee2f4cbf3883f36.