Filters cannot have many resource_types - each filter needs to have its own resource type, e.g: one filter does not accommodate permissions for "Host" and "DiscoveredHost".
This is enforced by a validator in the model, but our fixtures are able to completely skip it. The result is that plugins have Filters with multiple resource types. This has not been a problem until recently, as we added a test that checks that the db filter cache can be recreated (CacheManagerTest). On plugins, this test fails to pass as fixtures are wrongly created.