Django 1.8: Existing Migrations Files Erroring After FK Model Removed

[dy...@dylan-jenkinson.nz]

Hi there,

In one of our projects we have removed an old model that was no longer used. It was used as a FK on a couple of other models, so there are old migrations that have the model in them. I am trying to migrate into a fresh new DB to do some new work on the app. The migrations proceed happily until they hit this FK, where they then blow up with a ValueError: Related model 'x' cannot be resolved. I thought to try squishing the migrations to remove the need for that FK migration, but that does not seem to have helped. I suspect that this is related to the fact that the model is in a different application, and maybe that means that the squishing cannot figure out that it is no longer there?

What I would like to know is how other people approach this problem? How would you remove an existing but no longer used model from your system? Is there a recommended way of doing this?

Thanks for any help or advice.

Dylan

[Tim Graham]

It might be that you've made a mistake such as importing a model for RunPython rather than using apps.get_model(). Could you share the traceback?

[dy...@dylan-jenkinson.nz]

Hi Tim,

Traceback is below.

As a temporary measure to get me going I have commented out the existing migrations that added and removed the migrations. There is an initial migration that adds the FK, a RunPython that checks for the FK to perform some logic, and a migration to remove the FK. The RunPython does not import the model that has the FK. It uses the apps.get_model() method. I have also added the function called by RunPython below, in case that is of any use.

Thanks,

Dylan

Traceback:

Traceback (most recent call last):

  File "/home/dev/Programs/pycharm-2016.1.4/helpers/pycharm/django_manage.py", line 41, in <module>

    run_module(manage_file, None, '__main__', True)

  File "/app/python3/lib/python3.5/runpy.py", line 196, in run_module

    return _run_module_code(code, init_globals, run_name, mod_spec)

  File "/app/python3/lib/python3.5/runpy.py", line 96, in _run_module_code

    mod_name, mod_spec, pkg_name, script_name)

  File "/app/python3/lib/python3.5/runpy.py", line 85, in _run_code

    exec(code, run_globals)

  File "/wip/projects/progress/base/manage.py", line 10, in <module>

    execute_from_command_line(sys.argv)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/core/management/__init__.py", line 354, in execute_from_command_line

    utility.execute()

  File "/wip/environments/progress/lib/python3.5/site-packages/django/core/management/__init__.py", line 346, in execute

    self.fetch_command(subcommand).run_from_argv(self.argv)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/core/management/base.py", line 394, in run_from_argv

    self.execute(*args, **cmd_options)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/core/management/base.py", line 445, in execute

    output = self.handle(*args, **options)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/core/management/commands/migrate.py", line 222, in handle

    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/migrations/executor.py", line 110, in migrate

    self.apply_migration(states[migration], migration, fake=fake, fake_initial=fake_initial)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/migrations/executor.py", line 148, in apply_migration

    state = migration.apply(state, schema_editor)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/migrations/migration.py", line 115, in apply

    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/migrations/operations/models.py", line 59, in database_forwards

    schema_editor.create_model(model)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 236, in create_model

    definition, extra_params = self.column_sql(model, field)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/backends/base/schema.py", line 135, in column_sql

    db_params = field.db_parameters(connection=self.connection)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/models/fields/related.py", line 2010, in db_parameters

    return {"type": self.db_type(connection), "check": []}

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/models/fields/related.py", line 2001, in db_type

    rel_field = self.related_field

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/models/fields/related.py", line 1904, in related_field

    return self.foreign_related_fields[0]

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/models/fields/related.py", line 1638, in foreign_related_fields

    return tuple(rhs_field for lhs_field, rhs_field in self.related_fields if rhs_field)

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/models/fields/related.py", line 1625, in related_fields

    self._related_fields = self.resolve_related_fields()

  File "/wip/environments/progress/lib/python3.5/site-packages/django/db/models/fields/related.py", line 1610, in resolve_related_fields

    raise ValueError('Related model %r cannot be resolved' % self.rel.to)

ValueError: Related model 'workflow.Event' cannot be resolved

RunPython function:

def load_email_sent_workflow(apps, schema_editor):

    if schema_editor.connection.alias == 'default':

        db_alias = schema_editor.connection.alias

        EmailSent = apps.get_model('common', 'EmailSent')

        for email_sent in EmailSent.objects.using(db_alias).exclude(event__isnull=True):

            if email_sent.event:

                email_sent.workflow = email_sent.event.workflow

                email_sent.save(update_fields=['workflow'])

[Tim Graham]

I'm not sure. It looks like the crash happens when creating the model in the migration, not during the RunPython function. Could you create a minimal project that reproduces the crash so I can step through it?