filter using join model on has_many through

filter.rb

class Person
  has_many :accounts
  has_many :computers, through: :accounts
end

class Account
  belongs_to :person
  belongs_to :computer

  scope :administrators, -> { where(role: 'administrator') }
end

class Computer
  has_many :accounts
  has_many :people, through: :accounts
end

person.accounts.administrators.map(&:computer)  # ewww
person.computers.merge(Account.administrators)  # very nice!