Using active record's #merge to separate concerns

We’ve all written this kind of code at some point of our lives, a query that requires a join, and adding a condition to the joined table to limit the returned results, but then by doing that you’re adding a dependency, and you are coupling the two models together. Let’s explain this on an example to make it clear, assume we have a dvd renting store, and we have a system to manage the users, and dvds rentals, here’s how the models could look like ( in a very basic form ) This is a basic simple relation, let’s assume the dvds table has a simple flag that says which dvds were returned and which were not.
To find users that have un returned dvds, we could create a scope like this This scope has a problem, the thing is that the User class knows the name of the field (returned), and the value it wants it to be (false), if for any reason we decided to split the rentals into a separate table, or change the field’s name or value we need to modify this method too, which resides in the User class, it should not have that dependency.

To fix this we could split this condition into a scope in the dvds table Now the Dvd model only knows what unreturned means, to use that in the users method we will update the scope to use #merge It will generate the same SQL query, but without sharing the knowledge of the field or the value with the User model