Migrations for auth fails on Django 3.1: KeyError: 'deferrable'

I started a fresh project with Django 3.1. and the latest django-mssql-backend. Running python manage.py migrate auth fails on the “0008” migration:

  Applying auth.0008_alter_user_username_max_length...Traceback (most recent call last):                           File "manage.py", line 22, in <module>
    main()
  File "manage.py", line 18, in main                                                                                 execute_from_command_line(sys.argv)
...
  File "/.../lib/python3.6/site-packages/django/db/backends/base/schema.py", line 572, in alter_field
    old_db_params, new_db_params, strict)
  File "/.../lib/python3.6/site-packages/sql_server/pyodbc/schema.py", line 479, in _alter_field
    self.execute(self._create_unique_sql(model, columns=[old_field.column]))
  File "/.../lib/python3.6/site-packages/sql_server/pyodbc/schema.py", line 863, in execute
    sql = str(sql)
  File "/.../venv/lib/python3.6/site-packages/django/db/backends/ddl_references.py", line 200, in __str__
    return self.template % self.parts
KeyError: 'deferrable'

While experimenting a little, downgrading to Django 2.2 fixed the issue with the problematic migrations, and after upgrading back to 3.1 I was able to run the remaining migrations (those that are new in Django 3). Digging into the code base the problem seems quite straightforward: the django.db.backends.ddl_references.Statement seems indeed to expect a deferrable keyword argument, and after hacking the source code so that an empty string was given as deferrable, the migrations worked just fine. Unfortunately I’m not familiar with mssql or this library, and can’t simply attempt a pull request for this.

[franzejr]

I confirm downgrading to Django 3.0.10 fixes the issue.

[OskarPersson]

Closed in #86