has_many_through.rb

# Example of using 
require 'sqlite3'
require 'active_record'
ActiveRecord::Base.establish_connection("sqlite3::memory:")
conn = ActiveRecord::Base.connection
conn.create_table :users do |t|
  t.string :name
end
conn.create_table :user_matches do |t|
  t.references :sender
  t.references :target
  def to_s; "#{sender}->#{target}"; end
end
class UserMatch < ActiveRecord::Base
  belongs_to :target, class_name: 'User'
  belongs_to :sender, class_name: 'User'
end
class User < ActiveRecord::Base
  has_many :incoming_user_matches, class_name: 'UserMatch', foreign_key: :target_id
  has_many :outgoing_user_matches, class_name: 'UserMatch', foreign_key: :sender_id
  has_many :senders, through: :incoming_user_matches
  has_many :targets, through: :outgoing_user_matches
  def to_s; name; end
end
alice = User.create(name: 'Alice')
bob = User.create(name: 'Bob')
cecil = User.create(name: 'Cecil')

bob.targets << alice
alice.targets << cecil
cecil.targets << bob

UserMatch.all.each { |um| puts "User Match: #{um}" }
User.all.each do |u|
  puts "#{u} is targeting: #{u.targets.map(&:to_s).join(',')}"
  puts "#{u} is targeted by: #{u.senders.map(&:to_s).join(',')}"
end