change transactional primary key to custom ID for better portability

db/migrate/20170403174111_create_sale_audits.rb

@@ -1,7 +1,9 @@
 class CreateSaleAudits < ActiveRecord::Migration[5.0]
   def change
-    create_table :sale_audits do |t|
-      t.references :sale, foreign_key: true
+    create_table :sale_audits, :id => false, :primary_key => :sale_audit_id do |t|
+      t.string :sale_audit_id, :limit => 16, :null => false, :index => true, :unique => true #custom primary key - to ensure consistence for cloud syncing
+
+      t.string :sale_id, foreign_key: true, :limit => 16
       t.string :action, :null => false
       t.datetime :action_at, :null => false
       t.string :action_by, :null => false