title: validation for hash column in rails
body: <p>i am having a Users table (id,name,...)
   and a user_details table(id,user_id,additional_details,designation)</p>
  
  <p>where this additional_details is a hash with (:empid,:prjt)</p>
  
  <p>I am trying to add validations for the additional_details[:empid] and designation column. How to do that ??</p>