Django Auth : How are many-to-many relationship models created?

Question

django.contrib.auth.models have only 3 models, User, Group, Permission, as per the doc and code.

Doc does say groups and permissions are many-to-many relationships.

But when we migrate how come other relations user_group, user_permissions, group_permissions are created?

Answer 1

The tables for ManyToMany relationship are created by lines with models.ManyToManyField string that are in the migration file.

In this case they are created by the initial migration django/contrib/auth/migrations/0001_initial.py. It is very similar to django/contrib/auth/models.py.

The related SQL commands can be read by

$ python manage.py sqlmigrate auth 0001_initial

See the lines:
CREATE TABLE "auth_user_groups"...
CREATE TABLE "auth_group_permissions"...
CREATE TABLE "auth_user_user_permissions"...

Especially interesting two texts are around the words REFERENCES. That are the foreign keys to base tables, e.g.:
... "user_id" integer NOT NULL REFERENCES "auth_user" ("id") ...

---

EDIT:

You probably know a ManyToMany relationship joined by an explicit intermediate model specified by the parameter through=SomeModel with an extra field.

I explain it in example how you can use a hidden intermediate table in your code.

Imagine that you want to make a note to some permission why you added it to some person.

You can explore the model structure from command line, because in "auth/models.py" you could read only the name "user_parmissions".

>>> User._meta.local_many_to_many
[<django.db.models.fields.related.ManyToManyField: groups>,
 <django.db.models.fields.related.ManyToManyField: user_permissions>]
>>> User.user_permissions.through
<class 'django.contrib.auth.models.User_user_permissions'>
>>> User.user_permissions.through._meta.local_fields
[<django.db.models.fields.AutoField: id>,
 <django.db.models.fields.related.ForeignKey: user>,
 <django.db.models.fields.related.ForeignKey: permission>]

your_app/models.py

from django.contrib.auth.models import User, Group, Permission
from django.db import models

# This line is instead of import, because the implicit intermediate table
# can not be imported.
UserPermission = User.user_permissions.through

class UserPermissionReason(models.Model)
    user_permission = models.ForeignKey(UserPermission, ...)
    reason = models.CharField(help_text="Why I added that permission", ...)

makemigrations, migrate, shell...

Example: Create a user permission with your note

>>> from your_app.models import UserPermissionReason, UserPermission
>>> from django.contrib.auth.models import User, Group, Permission
>>> permission = Permission.objects.get(name='Can add user',
                                        content_type__app_label='auth')
>>> permission
<Permission: auth | user | Can add user>
>>> user_permission = UserPermission.objects.create(user=me_superuser,
                                                    permission=permission)
>>> why = UserPermissionReason.objects.create(
        user_permission=user_permission,
        reason="Because I can :-)"
    )

If you don't need to reference the intermediate table you would use a normal simple some_ruser.user_permissions.add(permission)