MIgrations: Creating and then renaming a ForeignKey fails when index and constraint created

[Alexander Dutton]

Minimal test case at ​https://github.com/ox-it/create-rename-foreignkey-testcase; example Travis run at ​https://travis-ci.org/ox-it/create-rename-foreignkey-testcase/jobs/153535976.

The following migration fails:

    operations = [
        migrations.CreateModel(
            name='Planet',
            fields=[
                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
            ],
        ),
        migrations.CreateModel(
            name='Star',
            fields=[
                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
            ],
        ),
        migrations.AddField(
            model_name='planet',
            name='parent_star',
            field=models.ForeignKey(to='crft.Star'),
        ),
        migrations.RenameField(
            model_name='planet',
            old_name='parent_star',
            new_name='star',
        ),
    ]

as it tries to create the foreign key constraint and index at the end against the parent_star_id field, which has already been renamed to star_id.

This arose for as when I wanted to replace a ForeignKey with another to a different model, but perform a data migration from the old to the new. The idea was: create new foreign key with temporary name, migrate data, remove old foreign key, rename the new one to the original name.

I can get round it by first renaming the old field to a temporary name and creating the new one with the name we want by the end, but this causes the backwards migration to fail for the same reason.