Migration for M2M Field fails with FieldDoesNotExist when removing null=True from model

[Ronny Vedrilla]

I removed the null=True as suggested in django 1.8 warnings from my M2M field in the model.

django migrations created the following:

class Migration(migrations.Migration):

[...]
    operations = [
        migrations.AlterField(model_name='project', name='my_field', field=models.ManyToManyField(related_name='in_charge_projects', verbose_name='label', to='account.Employee', blank=True),
        ),
[...]
]

I get the following error:

"django.core.exceptions.FieldDoesNotExist: Project_my_field has no field named None"

[Andriy Sokolovskiy]

Can not reproduce this locally.
Please provide a test project, where this error will exists.

[codefisher]

I had exactly the same problem. The following is a stack trace for the error.

  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 338, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/__init__.py", line 330, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 390, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/base.py", line 441, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python2.7/dist-packages/django/core/management/commands/migrate.py", line 221, in handle
    executor.migrate(targets, plan, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/executor.py", line 110, in migrate
    self.apply_migration(states[migration], migration, fake=fake, fake_initial=fake_initial)
  File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/executor.py", line 147, in apply_migration
    state = migration.apply(state, schema_editor)
  File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/migration.py", line 115, in apply
    operation.database_forwards(self.app_label, schema_editor, old_state, project_state)
  File "/usr/local/lib/python2.7/dist-packages/django/db/migrations/operations/fields.py", line 201, in database_forwards
    schema_editor.alter_field(from_model, from_field, to_field)
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/base/schema.py", line 469, in alter_field
    return self._alter_many_to_many(model, old_field, new_field, strict)
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/sqlite3/schema.py", line 232, in _alter_many_to_many
    override_uniques=(new_field.m2m_field_name(), new_field.m2m_reverse_field_name()),
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/sqlite3/schema.py", line 137, in _remake_table
    self.create_model(temp_model)
  File "/usr/local/lib/python2.7/dist-packages/django/db/backends/base/schema.py", line 268, in create_model
    columns = [model._meta.get_field(field).column for field in fields]
  File "/usr/local/lib/python2.7/dist-packages/django/db/models/options.py", line 554, in get_field
    raise FieldDoesNotExist('%s has no field named %r' % (self.object_name, field_name))
django.core.exceptions.FieldDoesNotExist: Forum_subscriptions has no field named None

[codefisher]

Just looking noticed at ​https://docs.djangoproject.com/en/1.8/ref/models/meta/ there was a change to the get_field() method in 1.8. Maybe create_model is still assuming the old functionality of get_field() here? Maybe what is needed is something like this:

columns = [model._meta.get_field(field).column for field in fields if not field.many_to_many]

I have not tested, but am really just guessing at what looks right from the docs.

[Baptiste Mispelon]

Hi,

Could you provide us with the models you used as well?

Thanks.

[codefisher]

The failing migration: ​https://github.com/codefisher/djangopress/blob/master/djangopress/forum/migrations/0011_auto_20150426_1821.py
The models: ​https://github.com/codefisher/djangopress/blob/master/djangopress/forum/models.py

[Markus Holtermann]

Is this on the 1.8.x development branch or the 1.8.0 release. We fixed a few bugs in the upcoming 1.8.1 release: ​https://docs.djangoproject.com/en/dev/releases/1.8.1/

Specifically #24513 and #24573 come to mind.

[codefisher]

The 1.8.0 release as given by pip.
I guess this is a dup then. Did not find the other ones when I did a search for it.