Check out example codes for "getting particular data after populating". It will help you in understanding the concepts better.

Code Example 1

.lean().populate('user', 'email.address facebook.address')
  .exec(function (err, subscription){ 
    if (subscription.user.email.address) {
        delete subscription.user.facebook;
    } else {
        delete subscription.user.email;
    }
  });

Learn ReactJs, React Native from akashmittal.com