Question

嗨，我有像这样的mongodb查询。

.aggregate(
            [
                { $match : { comment_box : "somdata" } },

                { $project : {
                    comment : 1,                      
                        created_at: 1,
                        current_date: "$replies.created_at",
                    },
                    dateDifference: { $subtract: [ new Date(), "$created_at"] }
                }},
            ]
        )

想象一下，我想从当前日期字段中的回复数组中获取created_at值。但是这个查询返回

current_date: [
   "2016-03-08T13:48:27.882Z",
   "2016-03-08T14:26:22.194Z"
]

而不是此数组中每个元素的当前created_at值

我尝试了很多方法但却出错了

current_date: "$replies.0.created_at",
current_date: "$replies.$.created_at",
current_date: "$$replies.created_at",

等

请帮我检索这样的数据

current_date:"2016-03-08T13:48:27.882Z",

Answer 1

我假设你想要最新的评论日期。在这种情况下，您可以选择$max：

current_date: { $max: '$replies.created_at' }

演示：

> db.comments.insert({_id: 0, replies: [{created_at: new Date('2017-04-03')}, {created_at: new Date('2017-02-03')} ]})
WriteResult({ "nInserted" : 1 })
> db.comments.insert({_id: 1, replies: []})
WriteResult({ "nInserted" : 1 })
> db.comments.aggregate([{$project: {current_date: {$max: '$replies.created_at'}}}])
{ "_id" : 0, "current_date" : ISODate("2017-04-03T00:00:00Z") }
{ "_id" : 1, "current_date" : null }

请注意具有空replies数组的文档如何获得null。

MongoDB：聚合$ project获取数组当前项

1 个答案: