When a migration is performed where a ManyToMany field is added, the intermediary table is automatically created. However, when performing a backwards migration, this table is not deleted. Consider the following:
# locations/models.py
from products.models import Product
class Location(models.Model):
name = models.CharField(max_length=100)
products_sold = models.ManyToManyField(Product)
We create our initial migration for locations and run it:
$ ./manage.py makemigrations locations
Migrations for 'locations':
0001_initial.py:
- Create model Location
$ ./manage.py migrate locations
Operations to perform:
Apply all migrations: locations
Running migrations:
Applying locations.0001_initial... OK
Now, if we want to, we should be able to undo that migration and re-apply it. Often in development I re-do my initial migrations to keep things simple before I do my first deployment. Unfortunately when you re-aplly the migration we get an exception, because the backward migration didn't remove the intermediary table:
$ ./manage.py migrate locations zero
Operations to perform:
Unapply all migrations: locations
Running migrations:
Unapplying locations.0001_initial... OK
$ ./manage.py migrate locations
Operations to perform:
Apply all migrations: locations
Running migrations:
Applying locations.0001_initial...Traceback (most recent call last):
File "./manage.py", line 10, in <module>
execute_from_command_line(sys.argv)
File "/tmp/djangotest/src/django/django/core/management/__init__.py", line 427, in execute_from_command_line
utility.execute()
...
File "/tmp/djangotest/src/django/django/db/backends/utils.py", line 77, in execute
return super(CursorDebugWrapper, self).execute(sql, params)
File "/tmp/djangotest/src/django/django/db/backends/utils.py", line 61, in execute
return self.cursor.execute(sql, params)
File "/tmp/djangotest/src/django/django/db/utils.py", line 93, in __exit__
six.reraise(dj_exc_type, dj_exc_value, traceback)
File "/tmp/djangotest/src/django/django/db/backends/utils.py", line 61, in execute
return self.cursor.execute(sql, params)
django.db.utils.ProgrammingError: relation "locations_location_products_sold" already exists
I've attached an example project where this can be reproduced.
example project