Migration autodetector fails for models with field named _order, but not using Meta.order_with_respect_to

[Fabian Büchler]

The commit ​https://github.com/django/django/commit/aa4acc164d1247c0de515c959f7b09648b57dc42 introduced a new function ModelState.get_field in django.db.migrations.state.

This converts the field name _order to the one defined in options['order_with_respect_to'] automatically, which fails if the model has a field _order but isn't using Meta.order_with_respect_to.

That is the case for models generated by django-simple-history (​https://github.com/jazzband/django-simple-history) for models that are originally using Meta.order_with_respect_to: the resulting historical records model has only _order but is not using the Meta option.

This shows when running mange.py migrate or manage.py makemigrations:

$ ./manage.py makemigrations --dry-run
Waiting for port 'mysql:3306' timeout 1s (attempt 1/60)
Port 'mysql:3306' is open
Traceback (most recent call last):
  File "./manage.py", line 42, in <module>
    main()
  File "./manage.py", line 36, in main
    execute_from_command_line(sys.argv)
  File "/usr/local/lib/python3.8/site-packages/django/core/management/__init__.py", line 425, in execute_from_command_line
    utility.execute()
  File "/usr/local/lib/python3.8/site-packages/django/core/management/__init__.py", line 419, in execute
    self.fetch_command(subcommand).run_from_argv(self.argv)
  File "/usr/local/lib/python3.8/site-packages/django/core/management/base.py", line 373, in run_from_argv
    self.execute(*args, **cmd_options)
  File "/usr/local/lib/python3.8/site-packages/django/core/management/base.py", line 417, in execute
    output = self.handle(*args, **options)
  File "/usr/local/lib/python3.8/site-packages/django/core/management/base.py", line 90, in wrapped
    res = handle_func(*args, **kwargs)
  File "/usr/local/lib/python3.8/site-packages/django/core/management/commands/makemigrations.py", line 172, in handle
    changes = autodetector.changes(
  File "/usr/local/lib/python3.8/site-packages/django/db/migrations/autodetector.py", line 43, in changes
    changes = self._detect_changes(convert_apps, graph)
  File "/usr/local/lib/python3.8/site-packages/django/db/migrations/autodetector.py", line 189, in _detect_changes
    self.generate_altered_fields()
  File "/usr/local/lib/python3.8/site-packages/django/db/migrations/autodetector.py", line 928, in generate_altered_fields
    old_field = self.from_state.models[app_label, old_model_name].get_field(old_field_name)
  File "/usr/local/lib/python3.8/site-packages/django/db/migrations/state.py", line 689, in get_field
    self.options['order_with_respect_to']
KeyError: 'order_with_respect_to'

I believe this could be solved using a bit more defensive code, like:

    def get_field(self, field_name):
        if field_name == '_order' and 'order_with_respect_to' in self.options:
            field_name = self.options['order_with_respect_to']
        return self.fields[field_name]