Mongodb 如何检索mongo'；s受嵌套属性限制的文档

设想以下数据结构：

[
   {
     "_id": ...,
     "title": "Post #1",
     "date": "2019-01-01"
     "comments": [
         {
             "_id": ...,
             "text": "Comment #1 on #1",
             "date": "2019-01-01"
         }
     ]
   },
   {
    "_id": ...,
    "title": "Post #2",
    "date": "2019-02-01"
    "comments": [
        {
            "_id": ...,
            "text": "Comment #1 on #2",
            "date": "2019-02-01" // too old
        },
        {
            "_id": ...,
            "text": "Comment #2 on #2",
            "date": "2019-02-10"
        },
        {
            "_id": ...,
            "text": "Comment #3 on #2",
            "date": "2019-02-20"
        }
    ]
  },
  {
    "_id": ...,
    "title": "Post #3",
    "date": "2019-02-12"
    "comments": [
        {
            "_id": ...,
            "text": "Comment #1 on #3",
            "date": "2019-02-15"
        }
    ]
  },

   {
     "_id": ...,
     "title": "Post #4",
     "date": "2019-03-01"
     "comments": [] // none
   }
]

我想在帖子中展示3条最新评论

应该是：

• 评论#1对#3
• 评论#3对#2
• 评论#2对#2

输出应该类似于：

[
  {
    "_id": ...,
    "title": "Post #3",
    "date": "2019-02-12"
    "comments": [
        {
            "_id": ...,
            "text": "Comment #1 on #3",
            "date": "2019-02-15"
        }
    ]
  },
   {
    "_id": ...,
    "title": "Post #2",
    "date": "2019-02-01"
    "comments": [
        {
            "_id": ...,
            "text": "Comment #3 on #2",
            "date": "2019-02-20"
        },
        {
            "_id": ...,
            "text": "Comment #2 on #2",
            "date": "2019-02-10"
        }
    ]
  }
]

是否可以在Mongo上执行此类搜索？ 需要注意的重要一点是：“限制”是3，但它返回了2个文档/帖子，总共有3条评论。

试试这个：

db.collection_name.aggregate([{
    $unwind : "$comments"
},{
    $sort : {"date" : 1, "comments.date" : 1}
},{
    $limit : 3
},{
    $group :{
        _id : "$_id",
        title : {$first : "$title"},
        date : {$first : "$date"},
        comments : {$push : "$comments"},
    }
}])

---

1:“未知的组运算符“$post”。然后我试着用“post”代替2：字段“comments”必须是累加器对象它应该是

push

，而不是

post

。我已经更新了我的答案，请检查。是的，它有效。我只需要修复排序：

$sort:{“comments.date”：-1}