title: Mongodb multi document insert ignore custom duplicate field error
body: <p>I have to insert 3 recordset from array 1 already exists and 2 are new </p>
  
  <p>e.g:</p>
  
  <pre><code>db.products.insert(
     [
       { imagename: "pen1", qty: 21 },
       { imagename: "pen", qty: 20 },
       { imagename: "eraser", qty: 25 }
     ]
  )
  </code></pre>
  
  <p>Wherein "{ imagename: "pen", qty: 20 }"` already exists and has unique key 
   on field "imagename" in mongodb</p>
  
  <p>as for now none of them are getting inserted and throwing 
  err: 'E11000 duplicate key error index: mongotest.mothership.$imagename_1 dup</p>
  
  <p>any suggestion how to insert remaining two ignoring error in single go !</p>