如何在MongoDB模式中创建嵌套对象

我目前有一个用于保存用户数据的MongoDB文档的以下架构：

var userSchema =  new mongoose.Schema({
  username: {type: String, unique : true},
  password: {type: String},
  firstname: String,
  lastname: String,
  sketches: 
              [ 
                {name: String, 
                 sketch: Array}
              ]

sketches属性必须是一个数组对象，其中每个对象都将一个草图的名称与一个保存草图数据的数组相关联。由于某种原因，最终会按以下方式创建模式：

{
    "__v" : 1,
    "_id" : ObjectId("57c4d7693aa85cea2acf4d4d"),
    "firstname" : "test",
    "lastname" : "name",
    "password" : "password123",
    "sketches" : [ 
        {
            "sketch" : []
        }
    ],
    "username" : "testname"
}

我不确定在MongoDB中创建嵌套对象的正确格式，但我假设它与JSON相同。模式应如何构造以产生对象数组。

编辑：

从PUT请求插入文档的Web服务：

app.route("/addSketch/:username").put(function(req, res, next) {
  var user_name = req.params.username;
  User.findOne({username:user_name},function(err,foundObject){
    if(err){
      console.log("error");
      res.status(500).send();
    }
    else{
      if(!foundObject){
        res.status(404).send();
      }
      else{
        
        if(req.body.strokes && req.body.sketchName){
          var sketchObj = [];
          sketchObj[req.body.sketchName] = req.body.strokes;
          foundObject.sketches.push(req.body.sketchData);
        }
        foundObject.save(function(err,updatedObject){
          if(err){
            console.log(err);
            res.status(500).send();
          }
          else{
            res.send(updatedObject);
          }
        });
      }
    }

  });


  console.log('saving on server');
   var form = formidable.IncomingForm();

   console.log(form);
   console.log('the type of the request received is', (typeof req));

  form.parse(req, function(err, fields, files) {
      res.writeHead(200, {"content-type": "text/plain"});
      res.write('received upload:\n\n');
    var name = fields.name;
    var newSketch = new SavedSketch();
      newSketch.name = name;
      newSketch.sketchData =  fields.value;
      newSketch.save(function(err,savedObject){
        if(err){
               console.log(err);
               res.status(500).json({status:'failure'})
            }
            else{
              console.log("ID: " + fields.value.id + " strokeData:" + fields.value.strokes);
               res.json({status: 'success'});
            } 
      });

         res.end();
  });
  });