Javascript 与相互引用的对象一起使用时，Pull在mongo db中不起作用

希望使用pull从数组中删除文档，但pull后的结果与前面的结果相同

 var schema = new Schema({
 username: String,
 tasks: [{ type: Schema.Types.ObjectId, ref: "Task" }],
 });
 module.exports = mongoose.model("Task", schema);
 var schema2 = new Schema({
 title: String,
 creator: { type: Schema.Types.ObjectId, ref: "User" },
 });
 module.exports = mongoose.model("Task", schema2);

 User.findOneAndUpdate({_id: "xx"}, { $pull: {tasks: req.params.id} }, (err, data) => {
    if (err) {
        return res.status(500).json({ error: 'error in deleting address' });
    }

    console.log('data is ',data)
    // res.json(data);
});