arel - Want to find records with no associated records in Rails

Question

Consider a simple association...

class Person
   has_many :friends
end

class Friend
   belongs_to :person
end

What is the cleanest way to get all persons that have NO friends in ARel and/or meta_where?

And then what about a has_many :through version

class Person
   has_many :contacts
   has_many :friends, :through => :contacts, :uniq => true
end

class Friend
   has_many :contacts
   has_many :people, :through => :contacts, :uniq => true
end

class Contact
   belongs_to :friend
   belongs_to :person
end

I really don't want to use counter_cache - and I from what I've read it doesn't work with has_many :through

I don't want to pull all the person.friends records and loop through them in Ruby - I want to have a query/scope that I can use with the meta_search gem

I don't mind the performance cost of the queries

And the farther away from actual SQL the better...

Answer 1

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 })