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I'm running into an issue in Django 1.7 when attempting to write multiple migrations in a row. Here's the basic setup of the migrations:

  1. Initial schema migration to create the models for an app
  2. Data migration that calls loaddata on a specific fixture that contains one-time default data
  3. A new optional field was added to one of the models, so it's a schemamigration to add the field

If I generate the first migration, run it, generate the second, run it, and then add the new field, generate the third migration, and run it, everything is fine. However, if my database were on migration #1 and then I pulled down from a source repository, migration 2 would fail because it uses the models from models.py when calling loaddata rather than the models as of the time of that migration. It then produces the following error:

"Unknown column '[field]' in 'field list'"

In this case, [field] is the new field that I added for migration #3. The error makes sense, because my database doesn't have the new field yet but loaddata expects it to be there (even though the fixture doesn't reference the new field), but is there any way to make loaddata use the database at the time of the migration rather than the current state in models.py? Or are there any other ways to get around this issue?

Thanks.

Scott
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1 Answers1

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I ended up writing a hack to get around this for now, but I feel like there has to be a better way. Instead of calling loaddata in the migration, I now call this function:

def load_fixture_in_data_migration(apps, schema_editor, fixture_filename, migration_file):
"""
Load fixture data in data migrations without breaking everything
when the models change later on
"""
fixture_dir = os.path.abspath(os.path.join(os.path.dirname(migration_file), '../fixtures'))
fixture_file = os.path.join(fixture_dir, fixture_filename)

fixture = open(fixture_file, 'rb')
objects = serializers.deserialize('json', fixture, ignorenonexistent=True)
for obj in objects:
    ObjApp = apps.get_model(obj.object._meta.app_label, obj.object._meta.object_name)
    new_obj = ObjApp(pk=obj.object.pk)
    for field in ObjApp._meta.fields:
        setattr(new_obj, field.name, getattr(obj.object, field.name))
    new_obj.save()
fixture.close()

And I call it like this from the data migration:

load_fixture_in_data_migration(apps, schema_editor, 'initial_add_ons.json', __file__)

Does anyone know a better way to do this? It feels really like a hack since I have to access object meta data to accomplish this.

Scott
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