AttributeError: 'NoneType' object has no attribute 'is_relation' - None is undefined

[Python Force]

Hello,

Was going from Django version 1.10.7 to 1.11.4 and my migrations were working in 1.10.7 and stopped in 1.11.4. All my migrations were migrated before the upgrade. I went back to 1.10.7 and it was working again. Going back to 1.11.4 same error.

Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "[...]/venv/lib/python2.7/site-packages/django/core/management/__init__.py", line 363, in execute_from_command_line
    utility.execute()
  File "[...]/venv/lib/python2.7/site-packages/django/core/management/__init__.py", line 355, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "[...]/venv/lib/python2.7/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "[...]/venv/lib/python2.7/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "[...]/venv/lib/python2.7/site-packages/django/core/management/commands/migrate.py", line 163, in handle
    pre_migrate_state = executor._create_project_state(with_applied_migrations=True)
  File "/[...]/venv/lib/python2.7/site-packages/django/db/migrations/executor.py", line 81, in _create_project_state
    migration.mutate_state(state, preserve=False)
  File "[...]/venv/lib/python2.7/site-packages/django/db/migrations/migration.py", line 92, in mutate_state
    operation.state_forwards(self.app_label, new_state)
  File "[...]/venv/lib/python2.7/site-packages/django/db/migrations/operations/fields.py", line 150, in state_forwards
    delay = not old_field.is_relation
AttributeError: 'NoneType' object has no attribute 'is_relation'

When I look in the fields.py file I can see on the line 139 a function:

def state_forwards(self, app_label, state):
    new_fields = []
    old_field = None
    for name, instance in state.models[app_label, self.model_name_lower].fields:
        if name != self.name:
            new_fields.append((name, instance))
        else:
            old_field = instance
    state.models[app_label, self.model_name_lower].fields = new_fields
    # Delay rendering of relationships if it's not a relational field
    delay = not old_field.is_relation
    state.reload_model(app_label, self.model_name_lower, delay=delay)

Found out that it was crashing on 1 of the model fields that are not existing anymore in the database. I added a line to print the name of the model and field to find out what is causing the crash.

If old_field = None then it is calling a method on empty variable and it throws and error.

Added this line

print app_label + " " + self.model_name_lower + " " + self.name

def state_forwards(self, app_label, state):
    new_fields = []
    old_field = None
    print app_label + " " + self.model_name_lower + " " + self.name
    for name, instance in state.models[app_label, self.model_name_lower].fields:
        if name != self.name:
            new_fields.append((name, instance))
        else:
            old_field = instance
    state.models[app_label, self.model_name_lower].fields = new_fields
    # Delay rendering of relationships if it's not a relational field
    delay = not old_field.is_relation
    state.reload_model(app_label, self.model_name_lower, delay=delay)

The Output was

cars video subtitle_url
cars photos gear
cars news tags
Traceback (most recent call last):
  File "manage.py", line 10, in <module>
    execute_from_command_line(sys.argv)
  File "[...]/venv/lib/python2.7/site-packages/django/core/management/__init__.py", line 363, in execute_from_command_line
    utility.execute()
  File "[...]/venv/lib/python2.7/site-packages/django/core/management/__init__.py", line 355, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "[...]/venv/lib/python2.7/site-packages/django/core/management/base.py", line 283, in run_from_argv
    self.execute(*args, **cmd_options)
  File "[...]/venv/lib/python2.7/site-packages/django/core/management/base.py", line 330, in execute
    output = self.handle(*args, **options)
  File "[...]/venv/lib/python2.7/site-packages/django/core/management/commands/migrate.py", line 163, in handle
    pre_migrate_state = executor._create_project_state(with_applied_migrations=True)
  File "/[...]/venv/lib/python2.7/site-packages/django/db/migrations/executor.py", line 81, in _create_project_state
    migration.mutate_state(state, preserve=False)
  File "[...]/venv/lib/python2.7/site-packages/django/db/migrations/migration.py", line 92, in mutate_state
    operation.state_forwards(self.app_label, new_state)
  File "[...]/venv/lib/python2.7/site-packages/django/db/migrations/operations/fields.py", line 150, in state_forwards
    delay = not old_field.is_relation
AttributeError: 'NoneType' object has no attribute 'is_relation'

That means it crashed on App[Cars] Model[News] and Field[Tags] - tags were not in the DB anymore long time ago. I found this migration:

migrations.RemoveField(
    model_name='news',
    name='tags',
),

And deleted it.

After that the migration ran 100% and I am fully functional on 1.11.4.

For the record the function in 1.10.7 is - and that is working

def state_forwards(self, app_label, state):
     new_fields = []
     for name, instance in state.models[app_label, self.model_name_lower].fields:
         if name != self.name:
             new_fields.append((name, instance))
     state.models[app_label, self.model_name_lower].fields = new_fields
     state.reload_model(app_label, self.model_name_lower)

1. fields.py

​https://github.com/django/django/blob/master/django/db/migrations/operations/fields.py

2. Who was working on it

​https://github.com/django/django/commit/45ded053b1f4320284aa5dac63052f6d1baefea9

Hope that helps.

Radek

[Tim Graham]

Looks like a duplicate of #28073. If you can provide steps to reproduce such as a sample project, please reopen that ticket.