title: Check empty fields on mongoose
body: <p>I have this function on my node REST API </p>
  
  <pre><code>var router = express.Router();
  
  /*router.get('/getUsers', function(req, res) {
      res.json({ message: 'hooray! welcome to our api!' });
  });*/
  
  router.route('/Users')
    .get(function(req,res){
        User.find(function(err,users){
          if(err)
            res.send(err);
  
          console.log('GET request');
          res.json(users);
        });
  })
    .post(function(req,res){
        var user = new User();
        user.name = req.body.name;
        user.lastName = req.body.lastName;
  
        if(!user.name || !user.lastName){
          user.save(function(err){
            if(err)
              res.send(err);
  
            console.log('POST request');
            res.json({message:'Usuario creado'});
          });
        }else{
          res.json({message:'Campos vacíos'});
        }
  
    });
  </code></pre>
  
  <p>It works but when I a send a POST  request with empty parameters it saves a document in mongoDB without the <code>name</code> and <code>lastName</code> fields and <code>if(!user.name || !user.lastName)</code> doesn't work to avoid it</p>