[Django] #33914: App level default_auto_field is ignored for m2m through tables

[Django]

#33914: App level default_auto_field is ignored for m2m through tables
-------------------------------------+-------------------------------------
               Reporter:  Borislav   |          Owner:  nobody
  Ivanov                             |
                   Type:             |         Status:  new
  Uncategorized                      |
              Component:             |        Version:  3.2
  Migrations                         |
               Severity:  Normal     |       Keywords:  default_auto_field
           Triage Stage:             |      Has patch:  0
  Unreviewed                         |
    Needs documentation:  0          |    Needs tests:  0
Patch needs improvement:  0          |  Easy pickings:  0
                  UI/UX:  0          |
-------------------------------------+-------------------------------------
 When default_auto_field is defined on app level, it is ignored for
 m2m through tables. This effectively defaults them to what is defined
 in DEFAULT_AUTO_FIELD setting.

 This happens in two different code paths, ultimately affecting
 Options::_get_default_pk_class:

 1. When AppConfigStub is supplied instead of AppConfig it is stripped from
 most of the properties including default_auto_field.
 2. When the first migration for a given app is applied, we don't get any
 app config as it is lazily registered in StateApps::register_models. As
 this is the first model for this app, the config is no there yet.

 Here's modified configuration for polls app that demonstrates the issue:

 mysite/settings.py:
 {{{#!python
 ...
 DATABASES = {
     'default': {
         'ENGINE': 'django.db.backends.postgresql',
         'NAME': 'empty',
     }
 }
 ...
 DEFAULT_AUTO_FIELD = 'django.db.models.AutoField'
 }}}

 mysite/polls/apps.py:
 {{{#!python
 ...
 class PollsConfig(AppConfig):
     default_auto_field = 'django.db.models.BigAutoField'
     name = 'polls'
 }}}

 mysite/polls/models.py:
 {{{#!python
 ...
 class KlassA(models.Model):
     field_a = models.CharField(max_length=200)


 class KlassB(models.Model):
     field_b = models.CharField(max_length=200)
     m2m_to_a = models.ManyToManyField(
         KlassA, blank=True
     )

 }}}

 Then run `makemigrations` and `sqlmigrate polls 0001_initial`:
 {{{
 BEGIN;
 --
 -- Create model KlassA
 --
 CREATE TABLE "polls_klassa" ("id" bigserial NOT NULL PRIMARY KEY,
 "field_a" varchar(200) NOT NULL);
 --
 -- Create model KlassB
 --
 CREATE TABLE "polls_klassb" ("id" bigserial NOT NULL PRIMARY KEY,
 "field_b" varchar(200) NOT NULL);
 CREATE TABLE "polls_klassb_m2m_to_a" ("id" serial NOT NULL PRIMARY KEY,
 "klassb_id" bigint NOT NULL, "klassa_id" bigint NOT NULL);
 ALTER TABLE "polls_klassb_m2m_to_a" ADD CONSTRAINT
 "polls_klassb_m2m_to_a_klassb_id_klassa_id_05b47d06_uniq" UNIQUE
 ("klassb_id", "klassa_id");
 ALTER TABLE "polls_klassb_m2m_to_a" ADD CONSTRAINT
 "polls_klassb_m2m_to_a_klassb_id_58ef37e1_fk_polls_klassb_id" FOREIGN KEY
 ("klassb_id") REFERENCES "polls_klassb" ("id") DEFERRABLE INITIALLY
 DEFERRED;
 ALTER TABLE "polls_klassb_m2m_to_a" ADD CONSTRAINT
 "polls_klassb_m2m_to_a_klassa_id_b63f26b4_fk_polls_klassa_id" FOREIGN KEY
 ("klassa_id") REFERENCES "polls_klassa" ("id") DEFERRABLE INITIALLY
 DEFERRED;
 CREATE INDEX "polls_klassb_m2m_to_a_klassb_id_58ef37e1" ON
 "polls_klassb_m2m_to_a" ("klassb_id");
 CREATE INDEX "polls_klassb_m2m_to_a_klassa_id_b63f26b4" ON
 "polls_klassb_m2m_to_a" ("klassa_id");
 COMMIT;
 }}}

 Here we get `CREATE TABLE "polls_klassb_m2m_to_a" ("id" serial NOT NULL
 PRIMARY KEY...`

 If we change in settings.py `DEFAULT_AUTO_FIELD` to
 `django.db.models.BigAutoField` we get `CREATE TABLE
 "polls_klassb_m2m_to_a" ("id" bigserial NOT NULL PRIMARY KEY...`

 I did a little bit of research and found out 2 probable causes affecting
 Options::_get_default_pk_class:

 1. When AppConfigStub is supplied instead of AppConfig it is stripped
 from most of the properties including default_auto_field.
 2. When the first migration for a given app is applied, we don't get
 any app config as it is lazily registered in StateApps::register_models.
 As this is the first model for this app, the config is no there yet.

-- 
Ticket URL: <https://code.djangoproject.com/ticket/33914>
Django <https://code.djangoproject.com/>
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