Migrating in Lily

We use the built-in Django migrations for both our schema & data migrations. For more detailed information you should take a look at the Django docs.

Caveats

Data migrations have two things you need to know about:

• The modified date
• Seperating schema and data changes

Modified date

It is important to disable the updating of modified fields on models during data migrations. The modified date should always represent the date a user last modified it.

Unfortunately as of writing, there is no way to set this behaviour automatically, so you’ll have to do it yourself using update_modified = False.

Practical example:

# -*- coding: utf-8 -*-
from __future__ import unicode_literals

from django.db import models, migrations


def data_migration_forward(apps, schema_editor):
    Account = apps.get_model('accounts', 'Account')

    for account in Account.objects.all():
        account.update_modified = False
        # Do some mutations on the account here
        account.save()  # Because update_modified is False, this will not update the modified date of the account.

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

    operations = [
        migrations.RunPython(data_migration_forward),
    ]

Seperation of schema/data changes

You should never combine a schema and data migration in a single migration file. This is because of consistency and error prevention. In some cases it is possible to combine the two, but you still shouldn’t do it.

Thus, on PostgreSQL, for example, you should avoid combining schema changes and RunPython operations
in the same migration or you may hit errors like OperationalError: cannot ALTER TABLE "mytable"
because it has pending trigger events.

As can be read on the Django docs somewhat near the end of the block.