Add counter for number of each product in order

app/models/order.rb

@@ -1,7 +1,15 @@
 class Order < ActiveRecord::Base
   belongs_to :user
   has_many :order_products
-  has_many :products, through: :order_products
+  has_many :products, -> { includes :order_product }, { through: :order_products} do
+    def << (product)
+      if proxy_association.owner.products.include?(product)
+        proxy_association.owner.order_products.find_by(product: product).increment!(:count, 1)
+      else
+        super
+      end
+    end
+  end
 
   default_scope -> { order('created_at DESC') }
 

app/models/product.rb

@@ -1,2 +1,3 @@
 class Product < ActiveRecord::Base
+  has_one :order_product
 end

db/migrate/20141128123152_add_products_count_to_order_products.rb

@@ -0,0 +1,5 @@
+class AddProductsCountToOrderProducts < ActiveRecord::Migration
+  def change
+    add_column :order_products, :count, :integer, default: 1
+  end
+end

db/schema.rb

@@ -11,11 +11,12 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20141125130447) do
+ActiveRecord::Schema.define(version: 20141128123152) do
 
   create_table "order_products", force: true do |t|
     t.integer "order_id"
     t.integer "product_id"
+    t.integer "count",      default: 1
   end
 
   create_table "orders", force: true do |t|