请教一个Mongoose问题

UserSchema = new Schema({

name: String,

password: String

})

var User = mongoose.model('User', UserSchema)

PostSchema = new Schema({

title: String,

body: String,

user: {type : Schema.ObjectId, ref : 'User'}

});

var Post = mongoose.model('Post', PostSchema);

app.get('/posts', function(req, res, next){

var user = new User({name: 'lentg', password: '123'});

var post = new Post({title: 'hello', body: 'world', user: user});

console.log(post.user); 这里输出为的用户是user._id， 在别的例子中输出的都是 user对象，想问一下这是怎么回事啊？

});