使用mongoose中的$ lookup对连接后的结果进行排序

Question

我使用$ lookup进行左连接，但我无法按帖子中的created_at字段对结果进行排序

我的查询

pipeline =  [{$match: { "_id" :  mongoose.Types.ObjectId(req.body.celebrity_id)}},
               {$lookup : { from : "posts" , localField : "_id" ,foreignField : "celebrity_id", as : "posts"}},
               ]`

User.aggregate( pipeline )
.exec(function(err , user){

    console.log(user)
    if(err){

      res.json({'success' : false , msg : 'Something went wrong please try again'});
    }else{

      res.json({'success' : true  , msg : 'success' , 'data' : user });
    }
})

现在我想按created_at字段

按降序对posts数组进行排序