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

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)')