Nah your chasing your tail there, all you wound need to do is nest the review routes inside the bathroom routes so that you would end up with /bathrooms/:bathroom_id/reviews/ 
if you tried to do `bathroom.review = Bathroom.find(params[:bathroom_id])` then you would be trying to assign a Bathroom object to another Bathroom objects review property/attribute `bathroom.review =` expects to have a Review object assigned to it.