title: 2 documents merge in MongoDB
body: <p>I need to get data from two documents from different collections(1 common field in them, no change to the db) in MongoDB. I am a newbie and please help me with this</p>
  
  <pre><code>a = db.users.find(username:'abc@xyz.com')
  b = db.tasks.find(username:'abc@xyz.com')
  </code></pre>
  
  <p>How to I get a variable c which has a and b merged?</p>
  
  <p>Please help. This may be trivial but help me.</p>