title: Rails: ActiveRecord custom average calculation
body: <p>Outside of using a raw SQL query, is there a nice way to divide an active record query result by a constant? A variable?</p>
  
  <p>Example:</p>
  
  <pre><code>Phones.group(:year).order(:year).average(:talk_time_min)
  </code></pre>
  
  <p>If I wanted to retrieve the <code>talk_time_min</code> in seconds, how could I do <code>talk_time_min / 60</code>?</p>