title: How group by and get the count in has_many :through rails
body: <p>Three models </p>
  
  <ul>
  <li>Tags</li>
  <li>Posts</li>
  <li>PostTags</li>
  </ul>
  
  <p>Tags has many Posts through PostTags</p>
  
  <p>so what i need is to get all the name of tags which has maximum number of posts with the count</p>