从汇总返回整个文档 [英] Return whole document from aggregation

问题描述

我正在使用以下查询为数据库中的每个帖子获取一条最新评论:

I'm using the following query to fetch one most recent comment for every post in database:

db.comments.aggregate([
    {
        "$match": {
            "post_id": {
                "$in": [ObjectId("52c5ce24dca32d32740c1435"), ObjectId("52c5ce24dca32d32740c15ad")]
            }
        }
     },
     {
         "$sort": {"_id": -1}
     },
     {
        "$group": {
            "_id": "$post_id",
            "lastComment": {
                "$first": "$_id"
            }
        }
     }
])

我希望它返回整个注释的文档，但它仅返回每个文档的_id字段.那么，将所有最近的注释作为一个整体文档(或至少包括其他一些字段)的正确方法是什么?

I expect it to return the whole comment's document but it only returns the _id field of each document. So what would be the proper way to get all most recent comments as a whole document (or at least include some other fields)?

推荐答案

当前，您无法通过单个$first运算符来获取整个comment文档.但是您可以在$group步骤中包括其他必填字段(类似于_id字段):

Currently you cannot get the whole comment document via single $first operator. But you can include other necessary fields (similar to _id field) during $group step:

{
    "$group": {
        _id: "$post_id",
        lastComment: { "$first": "$_id" },
        field_1: { "$first": "$field_1" },
        field_2: { "$first": "$field_2" },
        // ...
        field_N: { "$first": "$field_N" }
    }
}

---

根据此JIRA票证: https://jira.mongodb.org/browse/SERVER-5916 ，整个文档将可以从 2.5.3版本的汇总操作中返回.可以使用新变量:$$ROOT或$$CURRENT:

---

According to this JIRA ticket: https://jira.mongodb.org/browse/SERVER-5916, the whole document will be available to return from aggregation operations from 2.5.3 version. It will be possible using new variables: $$ROOT or $$CURRENT:

{
    "$group": {
        _id: "$post_id",
        lastComment: { "$first": "$$CURRENT" }
    }
}

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