在MongoDB Aggregation Pipeline中引用整个文档

Question

我可以使用'$'运算符引用MongoDB聚合管道中各个属性值的值.但是,如何访问(引用)整个文档？

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更新:提供解释方案的示例.

这是我正在尝试做的一个例子.我有一些推文.每条推文都有一个成员"群集",这表明特定推文所属的群集.

{
    "_id" : "5803519429097792069",
    "text" : "The following vehicles/owners have been prosecuted by issuing notice on the basis of photographs on dated... http://t.co/iic1Nn85W5",
    "oldestts" : "2013-02-28 16:11:32.0",
    "firstTweetTime" : "4 hours ",
    "id" : "307161122191065089",
    "isLoc" : true,
    "powertweet" : true,
    "city" : "new+delhi",
    "latestts" : "2013-02-28 16:35:05.0",
    "no" : 0,
    "ts" : 1362081807.9693,
    "clusters" : [
        {
            "participationCoeff" : 1,
            "clusterID" : "5803519429097792069"
        }
    ],
    "username" : "dtptraffic",
    "verbSet" : [
        "date",
        "follow",
        "prosecute",
        "have",
        "be"
    ],
    "timestamp" : "4 hours ",
    "entitySet" : [ ],
    "subCats" : {
        "Generic" : [ ]
    },
    "lang" : "en",
    "fns" : 18.35967,
    "url" : "url|109|131|http://fb.me/2CeaI7Vtr",
    "cat" : [
        "Generic"
    ],
    "order" : 7
} 

因为,我的收藏中有几十条推文,我想通过'clusters.clusterID'对所有推文进行分组.基本上,我想写一个如下查询:

db.tweets.aggregate (
{ $group : { _id : '$clusters.clusterID', 'members' : {$addToSet : <????> } } }
)

我想访问当前处理文档并在我放入上述查询的位置引用它.有谁知道怎么做？

Answer 1

在文档中,我发现该$$ROOT表达式解决了这个问题.

来自DOC:http: //docs.mongodb.org/manual/reference/operator/aggregation/group/#group-documents-by-author

Answer 2

我认为 MapReduce 对于这项任务更有用。

正如 Asya Kamsky 的评论中所写，我的示例对于 mongodb 来说不正确，请使用mongoDB 的官方文档。