嵌套数组上的 MongoDB $elemMatch 投影

Question

我有一个像这样的集合(摘要)。

    {
    "id":"summaryid",
    "locations": [
        {
            "id": "loc1",
            "datacenters": [
                {
                    "id": "dc1.1",
                    "clusters": [
                        {
                            "id": "cl1.1",
                            "servers": [
                                {
                                    "id": "srvr1.1",
                                    "services": [
                                        {
                                            "id": "srvc1.1"
                                        }
                                    ]
                                }
                            ]
                        }
                    ]
                },
                {
                    "id": "dc1.2",
                    "clusters": [
                        {
                            "id": "cl1.2",
                            "servers": [
                                {
                                    "id": "srvr1.2",
                                    "services": [
                                        {
                                            "id": "srvc1.2"
                                        }
                                    ]
                                }
                            ]
                        }
                    ]
                }
            ]
        },
        {
            "id": "loc2",
            "datacenters": [
                {
                    "id": "dc2.1",
                    "clusters": [
                        {
                            "id": "cl2.1",
                            "servers": [
                                {
                                    "id": "srvr2.1",
                                    "services": [
                                        {
                                            "id": "srvc2.1"
                                        }
                                    ]
                                }
                            ]
                        }
                    ]
                },
                {
                    "id": "dc2.2",
                    "clusters": [
                        {
                            "id": "cl2.2",
                            "servers": [
                                {
                                    "id": "srvr2.2",
                                    "services": [
                                        {
                                            "id": "srvc2.2"
                                        }
                                    ]
                                }
                            ]
                        }
                    ]
                }

            ]
        }
    ]
}

现在我只想要用于数据中心且 ID 为 dc1.1 的集群。我想为集群排除服务器。

我尝试使用带有 $elemMatch 和投影的查找查询，如下所示。

db.summary.find({}, {"locations": { $elemMatch: { "datacenters._id" : 
"dc1.1" } }, "locations.datacenters.clusters":0, 
"locations.datacenters.servers":0, "locations.datacentercount" : 0, 
"locations.clustercount" : 0, "locations.servercount" : 0}).pretty()

我仍在获取所有数据中心，而不仅仅是 1 个与 ID 匹配的数据中心。我不确定我是否做对了。

谢谢!

Answer 1

$elemMatch 无法投影嵌套数组元素。

您可以在 3.4 服务器中尝试以下聚合。

使用 $unwind 几次到达嵌套数组并应用 $match 来选择嵌套数组元素。

   db.summary.aggregate([
      {
        "$match": {
          "locations.datacenters._id": "dc1.1"
        }
      },
      {
        "$unwind": "$locations"
      },
      {
        "$unwind": "$locations.datacenters"
      },
      {
        "$match": {
          "locations.datacenters._id": "dc1.1"
        }
      },
      {
        "$project": {
          "locations.datacenters.clusters.servers": 0
        }
      }
    ])

{"$project": {"locations.datacenters.clusters.servers": 0}} 将删除 servers 字段，同时保留所有其他字段最终输出。

来自文档

If you specify the exclusion of a field other than _id, you cannot employ any other $project specification forms: i.e. if you exclude fields, you cannot also specify the inclusion of fields, reset the value of existing fields, or add new fields.

引用: https://docs.mongodb.com/manual/reference/operator/aggregation/project/#exclude-fields