title: Rails: Adding an index after adding column
body: <p>Suppose I created a table <code>table</code> in a Rails app. Some time later, I add a column running:</p>
  
  <pre><code>rails generate migration AddUser_idColumnToTable user_id:string. 
  </code></pre>
  
  <p>Then I realize I need to add <code>user_id</code> as an index. I know about the <code>add_index</code> method, but where should this method be called? Am I supposed to run a migration (if yes, which one ?), then adding by hand this method?</p>