title: How to select all of a unique attribute
body: <p>I would like to find all the Records that have unique email addresses. I am trying to perform this :</p>
  
  <pre><code>@uniq_referrals = []
  CardReferral.all.select{|a| @uniq_referrals &lt;&lt; a.email}
  @referral_count = @uniq_referrals.uniq.each{|a|CardReferral.find_by_email(a)}
  </code></pre>
  
  <p>But I would like to do it in a single call. Is that possible?</p>