title: How to map more than one Attribute with ActiveRecord?
body: <p>If I type in my console</p>
  
  <pre><code>u = User.first
  u.friends(&amp;map:username)
  </code></pre>
  
  <p>I get <code>["Peter", "Mary", "Jane"]</code> but I also want to show the birthday, so how do I do that? I tried </p>
  
  <pre><code>u.friends(&amp;map:username, &amp;map:birthday)
  </code></pre>
  
  <p>but this doesn't work.</p>