title: get count of has_many association in rails query
body: <p>I have a two <code>model</code>. <code>Post</code> and `Comment'.</p>
  
  <p><code>Post</code> has_may <code>comments</code>.</p>
  
  <p><strong>Query:</strong>
    I want to get the max count of comments in which post.</p>
  
  <p>This should be done by <code>rail</code> query.</p>