title: Mongodb group by multiple keys to get unique documents in collection
body: <p>I have collection and documents are structured in the following format:</p>
  
  <pre><code>[
   {
  
      "brand" : "Toshiba",
      "title" : "Toshiba Pors 7220CT / NW2",
      "category" : "notebooks",
      "code" : "ABCDTESTASD12",
      "pid" : "45790"
   }, 
   {
      "brand" : "Toshiba",
      "title" : "Toshiba Satellite Pro 4600 PIII800",
      "category" : "notebooks",
      "ean" : "PATDSRESSSN12",
      "pid" : "12345"
   }
  ]
  </code></pre>
  
  <p>Could you suggest me the query to find unique documents which have same brand,title,category,code so that I can see unique docs in collection. </p>