title: Generate new ObjectId using RoboMongo
body: <p>I have push an object in array of objects mongoose using robomongo</p>
  
  <pre><code>db.getCollection('model').update({_id:ObjectId('')},{$push : {array : {Object}}})
  </code></pre>
  
  <p>but this didnt created the ObjectId oin document. now i have to manually insert the ObjectId. Question is how to generate it and update the document</p>