Example Join Model

20140811094300_create_line_items.rb

class CreateLineItems < ActiveRecord::Migration
  def change
    create_table :line_items do |t|
      t.references :order
      t.references :product
      t.integer :quantity
      t.boolean :delivered
    end
  end
end

irb.rb

order = Order.last
product1 = Product.first

order.products << product1 # Creates the appropriate line_item for this relationship

product2 = Product.last
LineItem.create(order: order, product: product2, quantity: 4)

line_item.rb

class LineItem < ActiveRecord::Base
  belongs_to :product
  belongs_to :order
  
  before_create :set_default_quantity

  private

  def set_default_quantity
    @quantity ||= 1
  end
end

order.rb

class Order < ActiveRecord::Base

  has_many :line_items
  has_many :products, through: :line_items

end