When one needs to select a record in a table that do not have related records through a relation, it is generally suggested to use a subquery.
Personnaly, I prefer a high level approach, and I have a method to do it with scopes.
There are a few issues with the other solutions I originally used that I found from other wikis. I address the issues I had in this much simpler and shorter way. I am also going to explain what is going into way more detail than others to help people understand what's going on.