title: search on multiple collections with mongodb
body: <p>I want to check if particular data is present on multiple documents or not.</p>
  
  <p>eg I want to check if email exists in user collection and customer collection if it does then want to fetch the data.</p>
  
  <p>I have tried below query but it fails as it checks only one collection.</p>
  
  <pre><code>db.t_usermasters.aggregate([
  {
      $match: {
          "email" : "test@test.com"
      }
  },{
      $lookup: {
          from: "t_customermasters",
          localField: "email",
          foreignField: "email",
          as: "UserDetails"    
    }
  }
  ]).pretty();
  </code></pre>