为什么元素可以推送到数组但不能正确保存在mongodb中？

Question

我有一个文档，该文档具有不在架构中的数组属性。当我尝试通过推送元素来更新此属性时，文档在我的节点应用程序中得到更新：我可以看到对象已修改了数组，但是当文档保存时，数组将保持原样，并且其他属性也会更新。阅读代码时，您会很好理解。

User.findOne({email: req.user.email})
        .then(user=>{
            if(user.get("posts")){
                let posts = user.get("posts");
                posts.push("word");
                user.set("posts", posts);
                //the problem is here
                //`posts` is reassigned properly to `posts` with pushed value but it can't be saved with this changement.
            }else{
                //this is when the "posts" property doesn't exist in document so it just gets added well=>no problem
                user.set("posts", "word")
            }
            //if you have `user.set("a","b")` the document gets this field added and `posts` doesn't get updated
            //if the document has property "c" set to "d" and you have `user.set("c", "h")` the "c" property gets updated properly
            user.save();
            res.send(user);
            //you can see here that the object has been successfully updated but in mongodb you cannot see the `posts`updated
        })
        .catch(err=>{
            if(err)throw err;
        });

这是架构

const UserSchema = new Schema({
    _id: {type: ObjectId, auto: true},
    username: {type: String, required: true, max:18},
    email: {type: String, required: true},
    password: {type: String, required: true, min:5},
    date_of_registration: {type: Date, required: true}
}, { strict: false });

对不起，如果代码中有些重复。 预先感谢您的帮助。