Mongodb / Mongoose多次插入/更新条件

This is my Visitor schema:

这是我的访客模式：

_post: { type: Schema.Types.ObjectId, ref: 'posts' },    
_visitors: [{
   _user: { type: Schema.Types.ObjectId, unique: true, ref: 'users' },
   createdAt: { type: Date, default: Date.now }
}]
_post