title: inserting to mongodb collection
body: <p>I have some data in cart and then I'm appending user_id to it. When I do <code>console.log(cart)</code> and <code>console.log(data)</code> I get user_id along with all other keys but user_id is not saved in the cart collection. Why is this happening and how can I fix it     </p>
  
  <pre><code>cart.user_id = user_id
  console.log(cart);
  db.cart.insert(cart, function(err, data) {
     console.log(data);
     if (err) return next(err);
  })
  </code></pre>