coupons usages are counted with invoices

app/models/coupon.rb

@@ -1,5 +1,5 @@
 class Coupon < ActiveRecord::Base
-  has_many :invoice
+  has_many :invoices
 
   validates :code, presence: true
   validates :code, format: { with: /[A-Z0-9]+/ ,message: 'only caps letters and numbers'}

app/views/api/coupons/_coupon.json.jbuilder

@@ -1 +1,2 @@
-json.extract! coupon, :id, :name, :code, :percent_off, :valid_until, :max_usages, :active, :created_at
+json.extract! coupon, :id, :name, :code, :percent_off, :valid_until, :max_usages, :active, :created_at
+json.usages @coupon.invoices.count

app/views/api/coupons/index.json.jbuilder

@@ -0,0 +1,5 @@
+json.cache! [@coupons] do
+  json.array!(@coupons) do |coupon|
+    json.partial! 'api/coupons/coupon', coupon: coupon
+  end
+end

db/migrate/20160804073558_remove_usages_from_coupons.rb

@@ -0,0 +1,5 @@
+class RemoveUsagesFromCoupons < ActiveRecord::Migration
+  def change
+    remove_column :coupons, :usages, :integer
+  end
+end

db/schema.rb

@@ -11,7 +11,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema.define(version: 20160803104701) do
+ActiveRecord::Schema.define(version: 20160804073558) do
 
   # These are extensions that must be enabled in order to support this database
   enable_extension "plpgsql"
@@ -110,7 +110,6 @@ ActiveRecord::Schema.define(version: 20160803104701) do
     t.integer  "percent_off"
     t.datetime "valid_until"
     t.integer  "max_usages"
-    t.integer  "usages"
     t.boolean  "active"
     t.string   "stp_coupon_id"
     t.datetime "created_at",    null: false