# Movie Model
has_many :purchases
has_many :buyers, through: :purchases
# User Model
has_many :purchases, foreign_key: :buyer_id
has_many :movies, through: :purchases
# Purchase Model
belongs_to :movie
belongs_to :buyer, class_name: 'User'
http://www.sitepoint.com/build-online-store-rails/
How show movies list where purchases is nil?