FieldDoesNotExist during custom migration

[Gagaro]

Hello,

I'm trying to make a migration to change a field ("filter") from a ForeignKey to a CharField. The migration is as follow :

class Migration(migrations.Migration):

    operations = [
        migrations.AddField(
            model_name='Rendition',
            name='filter2',
            field=models.CharField(max_length=255, db_index=True),
        ),
        migrations.RunPython(forwards_data_migration, reverse_data_migration),
        migrations.RemoveField(model_name='Rendition', name='filter'),
        migrations.DeleteModel('Filter'),
        migrations.RenameField(model_name='Rendition', old_name='filter2', new_name='filter'),
    ]

When running the migration, I have the following error:

django.core.exceptions.FieldDoesNotExist: Rendition has no field named u'filter'

The error happens  during the RemoveField (migrations.RemoveField(model_name='Rendition', name='filter')).

So, a couple questions:

* Is there a better way to change a field type (with data migration)?

* Why do I have this error, when my field is present?

The migration can be found there: https://github.com/Gagaro/wagtail/blob/02f4804b49d6f8b65ffb193d7a0dfc7872d323bd/wagtail/wagtailimages/migrations/0014_remove_filter_model.py

And the relevant model is there: https://github.com/Gagaro/wagtail/blob/02f4804b49d6f8b65ffb193d7a0dfc7872d323bd/wagtail/wagtailimages/models.py#L458-L516

Thanks

[Gagaro]

The issue was with my model which had a unique_together with the field removed in it. Fixed migrations are:

class Migration(migrations.Migration):

    operations = [
        migrations.AddField(
            model_name='Rendition',
            name='filter2',
            field=models.CharField(max_length=255, db_index=True),
        ),
        migrations.RunPython(forwards_data_migration, reverse_data_migration),

        migrations.AlterUniqueTogether('rendition', set([])),

        migrations.RemoveField(model_name='Rendition', name='filter'),
        migrations.DeleteModel('Filter'),
        migrations.RenameField(model_name='Rendition', old_name='filter2', new_name='filter'),

        migrations.AlterUniqueTogether('rendition', set([('image', 'filter', 'focal_point_key')])),
    ]