change transactional primary key to custom ID for better portability

db/migrate/20170403140820_create_order_items.rb

@@ -1,7 +1,8 @@
 class CreateOrderItems < ActiveRecord::Migration[5.0]
   def change
-    create_table :order_items do |t|
-      t.references :order, foreign_key: true, :null => false
+    create_table :order_items, :id => false, :primary_key => :order_items_id do |t|
+      t.string :order_items_id, :limit => 16, :null => false, :index => true, :unique => true #custom primary key - to ensure consistence for cloud syncing
+      t.string :order_id, foreign_key: true, :null => false, :limit => 16
       t.string :order_item_status, :null => false, :default => "new"
       t.string :item_order_by #person who order this
       t.string :item_code, :null => false