Non-restful controllers - controllers without models / resources - would cause impressionist to throw an error on impression create, because it was unable to update the model counter cache due to lack of an impressionable_type and impressionable_id. An if statement around the counter-cache method on impressionable_type and impressionable_id solves this.
Test coverage around impressions for non-restful controller actions. Added dummy route, index method, view and dummy controller spec.