changing a ManyToManyField to a CharField while also deleting the model being connected to breaks migrations

[Michael D. Hoyle]

if you have model.py:

class Thing(models.Model):
  #fields

class ThingList(models.Model):
  things = models.ManyToManyField('Thing')

then run migrations, then change it to:

class ThingList(models.Model):
  things = models.CharField(max_length=200)

(note the removal of the Thing model)
and then make migrations, running the migrations fails with

  Applying brokenmigration.0002_auto_20141130_2034...Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/home/hoylemd/django-env/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 385, in execute_from_command_line
    utility.execute()
  File "/home/hoylemd/django-env/local/lib/python2.7/site-packages/django/core/management/__init__.py", line 377, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/home/hoylemd/django-env/local/lib/python2.7/site-packages/django/core/management/base.py", line 288, in run_from_argv
    self.execute(*args, **options.__dict__)
  File "/home/hoylemd/django-env/local/lib/python2.7/site-packages/django/core/management/base.py", line 338, in execute
    output = self.handle(*args, **options)
  File "/home/hoylemd/django-env/local/lib/python2.7/site-packages/django/core/management/commands/migrate.py", line 160, in handle
    executor.migrate(targets, plan, fake=options.get("fake", False))
  File "/home/hoylemd/django-env/local/lib/python2.7/site-packages/django/db/migrations/executor.py", line 63, in migrate
    self.apply_migration(migration, fake=fake)
  File "/home/hoylemd/django-env/local/lib/python2.7/site-packages/django/db/migrations/executor.py", line 97, in apply_migration
    migration.apply(project_state, schema_editor)
  File "/home/hoylemd/django-env/local/lib/python2.7/site-packages/django/db/migrations/migration.py", line 107, in apply
    operation.database_forwards(self.app_label, schema_editor, project_state, new_state)
  File "/home/hoylemd/django-env/local/lib/python2.7/site-packages/django/db/migrations/operations/fields.py", line 124, in database_forwards
    from_model = from_state.render().get_model(app_label, self.model_name)
  File "/home/hoylemd/django-env/local/lib/python2.7/site-packages/django/db/migrations/state.py", line 89, in render
    model=lookup_model,
ValueError: Lookup failed for model referenced by field brokenmigration.ThingList.things: brokenmigration.Thing

I have made a github repository demonstrating the behavior: ​https://github.com/hoylemd/djangomigrationsbug

I'm using version 1.7.1

[Markus Holtermann]

I classify this as a release blocker since it breaks a new feature introduced in 1.7.

[Markus Holtermann]

I attached an initial pull-request to work with: ​https://github.com/django/django/pull/3665

[Tim Graham <timograham@…>]

In 623ccdd598625591d1a12fc1564cf3ef9a87581f:

Fixed #23938 -- Added migration support for m2m to concrete fields and vice versa

Thanks to Michael D. Hoyle for the report and Tim Graham for the review.

[Tim Graham <timograham@…>]

In 1702bc52cc20ed0729893177fc8f4391b4b3183c:

[1.7.x] Fixed #23938 -- Added migration support for m2m to concrete fields and vice versa

Thanks to Michael D. Hoyle for the report and Tim Graham for the review.

Backport of 623ccdd598625591d1a12fc1564cf3ef9a87581f from master

[Tim Graham <timograham@…>]

In 79645529e7ce03a7eef1e51e7058a9880f83efb5:

Revert "[1.7.x] Fixed #23938 -- Added migration support for m2m to concrete fields and vice versa"

This reverts commit 1702bc52cc20ed0729893177fc8f4391b4b3183c.

This doesn't work on stable/1.7.x because #23844 wasn't backported and we're
not willing to do so because it's a large change.

[Tim Graham <timograham@…>]

In 2d638893651052dd8232911f392376c2cd707799:

Removed release note for refs #23938 as it couldn't be backported to 1.7.