Working example of searching self-joined, nested models with ransack. With Rails >4.0 join is made on the wrong association while on Rails 4.0 it works as expected

20150414115444_create_clients.rb

class CreateClients < ActiveRecord::Migration
  def change
    create_table :clients do |t|
      t.string :type
      t.references :party, index: true
      t.references :master_client, index: true

      t.timestamps
    end
  end
end

20150414115501_create_parties

class CreateParties < ActiveRecord::Migration
  def change
    create_table :parties do |t|
      t.string :name

      t.timestamps
    end
  end
end

README.md

Steps to reproduce the issue:

1. Clone this Gist in two environments, one running Rails 4.0, one running Rails 4.1/4.2.

2. Run bundle install and rake db:migrate (don't forget to change the Rails version in the Gemfile);

3. Open the rails console:

rails c

4. Create some test data. Here's an example:

p1 = Party.create(name: 'Party 1')
p2 = Party.create(name: 'Party 2')
mc = MasterClient.create(party: p1)
c = Client.create(party: p2, master_client: mc)

5. Issue a ransack search:

Client.ransack('master_client_party_name_cont' => 'arty').result

6. Observe the generated query:

a. On Rails 4.0, the query looks as expected:

SELECT "clients".* 
FROM "clients" 
  LEFT OUTER JOIN "clients" "master_clients_clients" ON "master_clients_clients"."id" = "clients"."master_client_id" AND "master_clients_clients"."type" IN ('MasterClient') 
  LEFT OUTER JOIN "parties" ON "parties"."id" = "master_clients_clients"."party_id" 
WHERE ("parties"."name" LIKE '%arty%')

b. On Rails 4.1/4.2, the last join is built the wrong way:

SELECT "clients".* 
FROM "clients" 
  LEFT OUTER JOIN "clients" "master_clients_clients" ON "master_clients_clients"."id" = "clients"."master_client_id" AND "master_clients_clients"."type" IN ('MasterClient') 
  LEFT OUTER JOIN "parties" ON "parties"."id" = "clients"."party_id" 
WHERE ("parties"."name" LIKE '%arty%')

client.rb

class Client < ActiveRecord::Base
  belongs_to :party
  belongs_to :master_client
end

Gemfile

gem 'rails', '4.0.0' # Then change this to '4.2.0'
gem 'ransack'

master_client.rb

class MasterClient < Client
end

party.rb

class Party < ActiveRecord::Base
  has_one :client
end