Update invoice.rb

replaced for loop for summation by .sum()

app/models/invoice.rb

@@ -31,23 +31,15 @@ def net_amount
   end
 
   def orders_sum
-    sum = 0
-    for order in orders 
-      sum += order.sum(:groups) 
-    end
-    sum
+    orders.sum { |order| order.sum(:groups) }
   end
 
   def orders_transport_sum
     orders.sum(:transport)
   end
 
   def deliveries_sum
-    sum = 0
-    for delivery in deliveries
-      sum += delivery.sum
-    end
-    sum
+    deliveries.sum(&:sum)
   end
 
   def expected_amount