[Django] #27768: makemigrations uses unnecessary AddField for ForeignKey depending on model name

[Django]

#27768: makemigrations uses unnecessary AddField for ForeignKey depending on 
model
name
--------------------------------------+------------------------
               Reporter:  Ed Morley   |          Owner:  nobody
                   Type:  Bug         |         Status:  new
              Component:  Migrations  |        Version:  master
               Severity:  Normal      |       Keywords:
           Triage Stage:  Unreviewed  |      Has patch:  0
    Needs documentation:  0           |    Needs tests:  0
Patch needs improvement:  0           |  Easy pickings:  0
                  UI/UX:  0           |
--------------------------------------+------------------------
 Using Django master, for this model `makemigrations` generates an
 inefficient migrations file, which uses a combination of `CreateModel` and
 `AddField`, rather than just using a single `CreateModel` operation.

 {{{#!python
 class Aaa(models.Model):
     pass

 class Foo(models.Model):
     fk = models.ForeignKey(Aaa,
 on_delete=django.db.models.deletion.CASCADE)
 }}}

 Resultant migration operations generated by `./manage.py makemigrations`:
 {{{#!python
     operations = [
         migrations.CreateModel(
             name='Foo',
             fields=[
                 ('id', models.AutoField(auto_created=True,
 primary_key=True, serialize=False, verbose_name='ID')),
             ],
         ),
         migrations.CreateModel(
             name='Zzz',
             fields=[
                 ('id', models.AutoField(auto_created=True,
 primary_key=True, serialize=False, verbose_name='ID')),
             ],
         ),
         migrations.AddField(
             model_name='foo',
             name='fk',
 field=models.ForeignKey(on_delete=django.db.models.deletion.CASCADE,
 to='testapp.Zzz'),
         ),
     ]
 }}}

 However if the `Zzz` model was instead called `Aaa`, then the migration
 file is correctly optimised (ie: the `ForeignKey` is defined within the
 `CreateModel` operation):
 {{{#!python
     operations = [
         migrations.CreateModel(
             name='Aaa',
             fields=[
                 ('id', models.AutoField(auto_created=True,
 primary_key=True, serialize=False, verbose_name='ID')),
             ],
         ),
         migrations.CreateModel(
             name='Foo',
             fields=[
                 ('id', models.AutoField(auto_created=True,
 primary_key=True, serialize=False, verbose_name='ID')),
                 ('fk',
 models.ForeignKey(on_delete=django.db.models.deletion.CASCADE,
 to='testapp.Aaa')),
             ],
         ),
     ]
 }}}

 Ideally the optimizer would either just use the order as defined in
 models.py (which would at least allow for users to order them sensibly),
 or else intelligently sort the models such that those with no (or fewer)
 foreign keys are listed in `operations` first, thereby reducing the number
 of cases where the FK has to be added in a separate `AddField` operation.

--
Ticket URL: <https://code.djangoproject.com/ticket/27768>
Django <https://code.djangoproject.com/>
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