title: Add autoincrement via migration in rails
body: <p>How to add an auto increment property to a column named <code>user_number</code> in my table <code>users</code> via Rails Migration.</p>
  
  <p>I already have an id field which is its primary key and it is an autoincrement field. Im trying to create a new autoincrement field without deleting this id field..</p>