title: Find by id or by object
body: <p>It's usal to see codes where a model <code>where</code> search in a foreign key is realized passing the whole object. For instance, search user exercises: </p>
  
  <pre><code>Exercise.where(user: user)
  </code></pre>
  
  <p>Another usual way to do that is:</p>
  
  <pre><code>Exercise.where(user_id: user.id)
  </code></pre>
  
  <p>Checking in console log, both querys search by the user id, but there is a "most recommended" way between both? If yes, why: more elegant, performatic?</p>