title: How to join multiple collections with $lookup in mongodb
body: <p>I want to join more than two collections in MongoDB using the aggregate <code>$lookup</code>. Is it possible to join? Give me some examples.</p>
  
  <p>Here I have three collections:</p>
  
  <p><code>users</code>:</p>
  
  <pre><code>{    
      "_id" : ObjectId("5684f3c454b1fd6926c324fd"),
      "email" : "admin@gmail.com",
      "userId" : "AD",
      "userName" : "admin"
  }
  </code></pre>
  
  <p><code>userinfo</code>:</p>
  
  <pre><code>{
      "_id" : ObjectId("56d82612b63f1c31cf906003"),
      "userId" : "AD",
      "phone" : "0000000000"
  }
  </code></pre>
  
  <p><code>userrole</code>:</p>
  
  <pre><code>{
      "_id" : ObjectId("56d82612b63f1c31cf906003"),
      "userId" : "AD",
      "role" : "admin"
  }
  </code></pre>