makemigrations uses unnecessary AddField for ForeignKey depending on model name

[Ed Morley]

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.

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:

    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):

    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.