From the oficial documentation:
For tests involving models with, it’s up to you to ensure the correct tables are created as part of the test setup.
I found a way. Modify the fixtures and add the SQL to generate the tables:
#0001_initial.py (or followings) class Migration(migrations.Migration): operations = [ migrations.RunSQL("CREATE TABLE..."), ... ]
I'm a "migrations newbie" so I don't know if this is the best option. But it works.