Want to find records with no associated records in Rails

Update 4 - Rails 6.1

Thanks to Tim Park for pointing out that in the upcoming 6.1 you can do this:

Person.where.missing(:contacts)

Thanks to the post he linked to too.

Update 3 - Rails 5

Thanks to @Anson for the excellent Rails 5 solution (give him some +1s for his answer below), you can use left_outer_joins to avoid loading the association:

Person.left_outer_joins(:contacts).where(contacts: { id: nil })

I've included it here so people will find it, but he deserves the +1s for this. Great addition!

Update 2

Someone asked about the inverse, friends with no people. As I commented below, this actually made me realize that the last field (above: the :person_id) doesn't actually have to be related to the model you're returning, it just has to be a field in the join table. They're all going to be nil so it can be any of them. This leads to a simpler solution to the above:

Person.includes(:contacts).where(contacts: { id: nil })

And then switching this to return the friends with no people becomes even simpler, you change only the class at the front:

Friend.includes(:contacts).where(contacts: { id: nil })

Update

Got a question about has_one in the comments, so just updating. The trick here is that includes() expects the name of the association but the where expects the name of the table. For a has_one the association will generally be expressed in the singular, so that changes, but the where() part stays as it is. So if a Person only has_one :contact then your statement would be:

Person.includes(:contact).where(contacts: { person_id: nil })

Original

Better:

Person.includes(:friends).where(friends: { person_id: nil })

For the hmt it's basically the same thing, you rely on the fact that a person with no friends will also have no contacts:

Person.includes(:contacts).where(contacts: { person_id: nil })

smathy has a good Rails 3 answer.

For Rails 5, you can use left_outer_joins to avoid loading the association.

Person.left_outer_joins(:contacts).where( contacts: { id: nil } )

Check out the api docs. It was introduced in pull request #12071.

This is still pretty close to SQL, but it should get everyone with no friends in the first case:

Person.where('id NOT IN (SELECT DISTINCT(person_id) FROM friends)')