Change relationship of invoices #390

This change will allow us to have invoices for more than one order/delivery
in the future. There are no UI changes for now.

db/migrate/20160217134742_change_invoice_relation.rb

@@ -0,0 +1,21 @@
+class ChangeInvoiceRelation < ActiveRecord::Migration
+  def up
+    add_column :deliveries, :invoice_id, :integer
+    execute 'UPDATE deliveries SET invoice_id = (SELECT id FROM invoices WHERE delivery_id = deliveries.id)'
+    remove_column :invoices, :delivery_id
+
+    add_column :orders, :invoice_id, :integer
+    execute 'UPDATE orders SET invoice_id = (SELECT id FROM invoices WHERE order_id = orders.id)'
+    remove_column :invoices, :order_id
+  end
+
+  def down
+    add_column :invoices, :delivery_id
+    execute 'UPDATE invoices SET delivery_id = (SELECT id FROM deliveries WHERE invoice_id = invoices.id)'
+    remove_column :deliveries, :invoice_id, :integer
+
+    add_column :invoices, :order_id
+    execute 'UPDATE invoices SET order_id = (SELECT id FROM orders WHERE invoice_id = invoices.id)'
+    remove_column :orders, :invoice_id, :integer
+  end
+end