Rails - has_many & belongs_to関係を既存のモデルに追加 ref: http://qiita.com/mnishiguchi/items/35e40ab46ef02d095ce0

20150725142417_add_room_to_movings.rb

class AddRoomToMovings < ActiveRecord::Migration
  def change
    add_reference :movings, :room, index: true, foreign_key: true
  end
end

20150725142845_add_moving_to_rooms.rb

class AddMovingToRooms < ActiveRecord::Migration
  def change
    add_reference :rooms, :moving, index: true, foreign_key: true
  end
end

20150725172522_create_moving_rooms.rb

class CreateMovingRooms < ActiveRecord::Migration
  def change
    create_table :moving_rooms do |t|
      t.integer :moving_id
      t.integer :room_id

      t.timestamps null: false
    end
  end
end

20150726014749_add_index_to_moving_rooms.rb

class AddIndexToMovingRooms < ActiveRecord::Migration
  def change
    add_index :moving_rooms, :moving_id
    add_index :moving_rooms, :room_id
    add_index :moving_rooms, [:moving_id, :room_id], unique: true
  end
end

file10.rb

class Moving < ActiveRecord::Base
  ...
  has_many :moving_rooms, dependent: :destroy
  has_many :rooms, through: :moving_rooms
  ...

file11.rb

class Moving < ActiveRecord::Base
  ...
  has_many :moving_rooms, dependent: :destroy
  has_many :rooms, through: :moving_rooms
  ...

file12.rb

class Room < ActiveRecord::Base
  ...
  has_many :moving_rooms, dependent: :destroy
  has_many :movings, through: :moving_rooms
  ...

file4.txt

$ rails destroy migration add_moving_to_rooms moving:references
      invoke  active_record
      remove    db/migrate/20150725142845_add_moving_to_rooms.rb

file5.txt

$ rails destroy  migration add_room_to_movings room:references
      invoke  active_record
      remove    db/migrate/20150725142417_add_room_to_movings.rb

file6.txt

$ bundle exec rake db:drop
$ bundle exec rake db:create
$ bundle exec rake db:migrate
$ bundle exec rake db:seed

file7.txt

rails g model MovingRoom moving_id:integer room_id:integer

moving.rb

class Moving < ActiveRecord::Base
  ...
  has_many :rooms, dependent: :destroy
  ...

moving_room.rb

class MovingRoom < ActiveRecord::Base
  belongs_to :moving
  belongs_to :room
end

room.rb

class Room < ActiveRecord::Base
  ...
  belongs_to :moving
  ...