order api - WIP & db structure change

app/models/customer.rb

@@ -5,5 +5,7 @@ class Customer < ApplicationRecord
   validates_presence_of :name, :contact_no
   validates :contact_no, uniqueness: true
 
-
+  def lastest_invoices
+    sales.where(:customer_id => self.id).order("created_at desc").limit(5)
+  end
 end