Populate method not returning documents - Mongoose

need help resolving this issue,I am trying to use the populate method to replace Id's with a reerenced document, but when I output the returned result, the documents are not returned, instead the Id values still remain. here is my schema and detail code to clearify, thanks

 var CommentSchema=new mongoose.Schema({
     comment:{type:String},
     accountId:{type:mongoose.Schema.ObjectId, ref:'Account'}
  });


var StatusSchema=new mongoose.Schema({
 comments:[CommentSchema],
 accountId:{type:mongoose.Schema.ObjectId, ref:'Account'},//should be replaced with docement
          status:{type:String}
      });

ACCOUNT SCHEMA

 var AccountSchema=new mongoose.Schema({
    email:{type:String,unique:true},
    password:{type:String},
    name:{
        first:{type:String},
        last:{type:String},
      full:{type:String}
    },
    contacts:[Contact],
    status:[Status]
  });

 var Account=mongoose.model('Account',AccountSchema);
  var Comment=mongoose.model('Comment',CommentSchema);
  var Status=mongoose.model('Status', StatusSchema);

//This is how I save new Comments

 app.post('/accounts/:id/status', function(req, res) {
    var accountId =  req.params.id;

    models.Account.findById(accountId, function(account) {
        account.save(function(err){
          if (err) throw err;
        var status=new models.Account.Status();
        status.accountId=accountId;
        status.status=req.param('status', '');
        status.save(function(err){
         account.status.push(status);
         account.save(function(err){
             if(err)console.log(err)
             else
                console.log('successful! status save...')
         });
        });
      });
   });
   res.send(200);
  });

//QUERY -I expect this query to replace the accountId with the referenced document, but when the query runs, accountId still holds the id rather than the referenced document

var getActivity= function(accountId,callback){
  Account.findOne({_id:accountId}).populate('Status.accountId').exec(function(err, items)    {
                  console.log('result is:'+items);
             });
      }