DataHub purge fails on some revisions

[rgrp]

See ​http://datahub.io/ckan-admin/trash and try to purge revisions (*not* datasets). It will fail on some of the revisions with errors like:

Problem purging revision 391db9e8-df57-4e0e-8fe6-d4e0c2318344: (IntegrityError?) update or delete on table "revision" violates foreign key constraint "group_extra_revision_revision_id_fkey" on table "group_extra_revision" DETAIL: Key (id)=(391db9e8-df57-4e0e-8fe6-d4e0c2318344) is still referenced from table "group_extra_revision". 'DELETE FROM revision WHERE revision.id = %(id)s' {'id': u'391db9e8-df57-4e0e-8fe6-d4e0c2318344'}