Scoping a Rails Model by an Attribute of Its Child

Today I ran into an interesting problem. I needed to scope a Rails model by checking whether it had a child with a certain attribute. The solution was simple but a little hard to find online, so I thought I’d share.

I started out with two models: Applicant and Application. Every applicant has_one application, and every application has a status. I want to create an :active scope for the Applicant class, in order to list only applicants whose application status is "in review". Here are the models.

Creating the Scope

It turned out creating the scope was simple. All I had to do was add one line to the Applicant class:

That’s it! The joins method creates a JOIN SQL statement on the applications table, returning the application whose applicant_id matches the ID of the applicant in question. That enables us to access the attributes of the applicant’s application to check if it is "in review" and return the applicant as part of the collection if so.

Do note that the joins method requires :application to be singular, while in the argument to the where method, it should be plural.