DjangoCon Europe 2016 - Don't be afraid of writing migrations

Speaker: Markus Holtermann (twitter) is a Django core developer with focus on the migrations framework.

Some things, makemigrations cannot do, and that's where you come in. Everything here is in in Postgres, should work in MySQL, SQLite unsure, Oracle … ask your support.

General

from django.db import migrations

class Migration(migrations.Migration)
    dependencies = []
    operations = []

• dependencies: other migrations that you depend on
• operations: all required operations
• migrations are run in correct order when applied and reverse order when unapplied

Recipe 1: Optimizing makemigrations output

Output is not always ordered cleverly, so that fields will be added in the very end, so you might want to change the operations around. Models are also processed in alphabetical order. (You could submit a patch here.)

Recipe 2: Adding a non-nullable column

First have a class with an initial data migration: forwards and backwards methods for generating and deleting two data points. Then, after migrating, you want to add another column which is nullable. Then just define a forwards method.

def forwards():
    for author in Authors.objects.all():
        # fill data


class Migration(migrations.Migration):
    operations = [
        migrations.RunPython(forwards, migrations.RunPython.noop)
    ]

Then we make the column non-nullable, and tell Django that we know what we are doing.

Recipe 3: Rename an App Without Dependencies

You must not have any incoming foreign keys which depend on this app.

To the model class, add this and migrate.

    class Meta:
        db_table = 'rename_app_here'

Then run migrate rename_app zero --fake to make Django think that you have not applied any migrations yet.

Then change the app name everywhere (settings.py etc.). Then fake-migrate back to the very front and delete the Meta attribute, and migrate.

Resources

We will get a GitHub repository with examples, check Markus' GitHub later.