title: Can mongoose model update Object Id field?
body: <p>Here is my code</p>
  
  <pre><code>setAssignee(req.payload.email, function(assignee){
  
      //assignee is now a Object Id that get from setAssignee function
  
      var value = {
          id : req.params.id,
          title : req.payload.title,
          description : req.payload.description,
          assignee : assignee._id,
          targetDate : req.payload.targetDate,
          updatedDate : new Date(),
      };
          var taskObj = new TaskModel(value);
  
          var query = { $set : value};
  
          taskObj.update({ id : req.params.id}, query, function(err){
              if(err) { return reply(err); };
              return reply({
                  id : req.params.id,
                  title : req.payload.title,
                  description : req.payload.description,
                  assignee : assignee,
                  targetDate : req.payload.targetDate,
                  updatedDate : new Date(),
              });
          });
      });
   };
  </code></pre>
  
  <p>setAssignee is a callback function to get objectId from other model. and I want to change it at 'assignee' field in my model.</p>
  
  <p>Here is var query log : </p>
  
  <pre><code>{ '$set': 
  { id: '10',
   title: 'task11124',
   description: 'description',
   assignee: 5471aad59c93d7dd010ce3c1,
   targetDate: '2014-11-29T20:15:34.000Z',
   updatedDate: Wed Nov 26 2014 11:23:05 GMT+0700 (ICT) } }
  </code></pre>
  
  <p>It unable to update. I think because of assignee: 5471aad59c93d7dd010ce3c1.
  Is it a correct format to update this objectId field ?</p>
  
  <p>Did it about assignee: 5471aad59c93d7dd010ce3c1 not have '' (quote) ?</p>
  
  <p>Or I miss something?</p>
  
  <p>Thanks a lot for help.</p>