Nullify deliveries and orders depending on an invoice (#718)

db/migrate/20181204000000_clear_invalid_invoices_from_orders.rb

@@ -0,0 +1,8 @@
+class ClearInvalidInvoicesFromOrders < ActiveRecord::Migration
+  class Order < ActiveRecord::Base; end
+  class Invoice < ActiveRecord::Base; end
+
+  def up
+    Order.where.not(invoice_id: Invoice.all).update_all(invoice_id: nil)
+  end
+end