限制$推送聚合

Question

我有这样的小组：

{
            "$group": {
                "_id": "$conversationId"
                "conversation": {
                    "$push": {"from": {
                            "firstName": "$fromObj.firstName",
                            "lastName": "$fromObj.lastName",
                            "title": "$fromObj.title",
                            "picture": "$fromObj.picture"

                        }
                        , "to": {
                            "firstName": "$toObj.firstName",
                            "lastName": "$toObj.lastName",
                            "title": "$toObj.title",
                            "picture": "$toObj.picture"
                        }
                        , "content": "$content",
                        "_id": "$_id",
                        "conversationId": "$conversationId",
                        "unreaded": "$unreaded"
                    }}

            }

        }

在$push内部，我需要推送最后5个项目，我该怎么做？如果可以直接将$ limit添加到$推送任何解决方案，那将是理想的吗？

Answer 1

您可以在使用$slice的$project之后向您的管道添加$group阶段，以便从累积的conversation数组中获取最后5个元素每个文档：

{$project: {conversation: {$slice: ['$conversation', -5]}}}