title: solr: group by multi valued field
body: <p>I have a solr doc structure</p>
  
  <pre><code>   [
       {
         category:{mobiles},
         name: iphone,
         store: chroma,
         discount: 10%
       },
      {
         category:{mobiles,laptops},
         name: Apple,
         store: Reliance Digitals,
         discount: 12%
       },{
         category:{electornics},
         name: Boss,
         store: chroma,
         discount: 5%
       },{
         category:{headphones,mobiles},
         name: Samsung,
         store: Universal,
         discount: 8%
       },{
         category:{headphones},
         name: HTC,
         store: Chroma,
         discount: 50%
       },
      ]
  </code></pre>
  
  <p>I need a solr query with the response</p>
  
  <pre><code>{
    headphones: 2
    mobiles: 3
    laptops: 1
    electornics: 1
  }
  </code></pre>
  
  <p>Simply, I need the group count over a multivalues field.</p>
  
  <p>Please help me with this.</p>
  
  <p>Thanks.</p>