mongodb：使用ObjectId进行聚合和$查找

Question

使用mongodb 3.4到MEAN，我有两个模式：

var userSchema = new mongoose.Schema({
email: {
    type: String,
    unique: true,
    required: true,
    trim: true },
username: {
    type: String,
    unique: true,
    required: true,
    trim: true },
password: {
    type: String,
    required: true }
},
{ collection: 'users'}
);
module.exports = mongoose.model('User', userSchema);

和另一个：

var followSchema = new mongoose.Schema({
username: {
   type: mongoose.Schema.Types.ObjectId,
   ref: 'User'
},
   followers: [{
   type: mongoose.Schema.Types.ObjectId,
   ref: 'User'
}],
following: [{
   type: mongoose.Schema.Types.ObjectId,
   ref: 'User'
}]
},{   collection: 'follows'}
);
module.exports = mongoose.model('Follow', followSchema);

当用户A有关注者并且跟随以下内容时，

在下面的对象中产生（作为例子）：

{
   "_id" : ObjectId("ae1d6f8..."),
   "username" : ObjectId("User A"),
   "followers" : [
       ObjectId("User B")
       ObjectId("User C")
    ],
   "following": [
       ObjectId("User B")
    ]
}

我想检索所有关注者的用户名并关注。我不工作的查询是：

Follow.aggregate([{
                $match: {
                    "username": user_id
                }
            },
                {
                    $lookup: {
                        "from": "User",
                        "localField": "following",
                        "foreignField": '_id',
                        "as": "userFollowing"
                    }
                },
                {
                    $lookup: {
                        "from": "User",
                        "localField": "followers",
                        "foreignField": '_id',
                        "as": "userFollowers"
                    }
                }, {
                    $project: {
                        "username": 1,
                        "userFollowers": 1,
                        "userFollowing": 1
                    }
                }
            ]).exec(function (err, doc) {
                console.log(doc);
                rest.json({
                    'state': true,
                    'msg': 'Follow list',
                    'doc': doc
                })
            })

这个空数组的结果以下是＆amp; userFollowers：

[ { _id: 5aeca47e600ff8f00617ac05,
    username: 5ae1ee337863273c0adac1d7,
    userFollowing: [],
    userFollowers: [] } ]

感谢您的帮助。