Add expected_amount to Invoice

expected_amount returns the sum of all associated orders together with
the transport costs.

app/models/invoice.rb

@@ -41,6 +41,24 @@ class Invoice < ApplicationRecord
     amount - deposit + deposit_credit
   end
 
+  def orders_sum
+    orders
+      .joins(order_articles: [:article_price])
+      .sum("COALESCE(order_articles.units_received, order_articles.units_billed, order_articles.units_to_order)" \
+        + "* article_prices.unit_quantity" \
+        + "* ROUND((article_prices.price + article_prices.deposit) * (100 + article_prices.tax) / 100, 2)")
+  end
+
+  def orders_transport_sum
+    orders.sum(:transport)
+  end
+
+  def expected_amount
+    return net_amount unless orders.any?
+
+    orders_sum + orders_transport_sum
+  end
+
   protected
 
   def valid_attachment