single method for updating cart item state / fixed problems with sinatra contrib

front-api/models/item_in_cart.rb

@@ -1,4 +1,20 @@
 class ItemInCart < ActiveRecord::Base
-  belongs_to :cart 
-  belongs_to :item 
-end
+  belongs_to :cart
+  belongs_to :item
+
+  validates_uniqueness_of :item_id, scope: :cart_id
+  validates :item_id, presence: true
+  validates :cart_id, presence: true
+
+  def self.update_state(cart_id, item_id, count)
+    item_in_cart = nil
+    ItemInCart.transaction do
+      item_in_cart = ItemInCart.where(cart_id: cart_id, item_id: item_id).first
+      item_in_cart ||= ItemInCart.new(cart_id: cart_id, item_id: item_id)
+      item_in_cart.count = count
+      item_in_cart.save!
+      item_in_cart.destroy! if count <= 0
+    end
+    return item_in_cart
+  end
+end