Question

Say I have 2 models:

class Person < ActiveRecord::Base
  has_many :addresses
end
class Address < ActiveRecord::Base
  belongs_to :person
end

I want to get all people who have exactly 2 addresses. Is there an easy ActiveRecord/Arel way of doing this with a subquery? I don’t want to use a counter_cache to do it.

Answer 1

This worked for me (using Rails 3, PostGreSQL):

Patient.joins(:interventions).group('patients.id').having('count(patient_interventions.id) = 2')

In your case, the following should return the Persons with exactly 2 Addresses:

Person.includes(:addresses)
      .select('persons.*')
      .group('persons.id')
      .having('count(addresses.id) = 2')
# Note use :joins instead of :includes if you don't want the addresses data