mongodb中嵌套文档中元素的计数

我有以下格式的数据

 {
        "result" : [ 
            {                    
                "advertiser" : "lordandtaylor"
                ,
                "networks" : [ 
                    "Pepperjam"
                ],
                "publishers" : [ 
                    "best-price.com", 
                    "allthingswholesale.com"
                ]
            }]
    }

{"advertiser":"lordandtaylor" , "networks":"1" , "publishers":"2"}

现在我想用下面的格式显示这个

 {
        "result" : [ 
            {                    
                "advertiser" : "lordandtaylor"
                ,
                "networks" : [ 
                    "Pepperjam"
                ],
                "publishers" : [ 
                    "best-price.com", 
                    "allthingswholesale.com"
                ]
            }]
    }

{"advertiser":"lordandtaylor" , "networks":"1" , "publishers":"2"}

我找到了解决办法

db.link.aggregate([
{
    {   
    $project : { _id: 0, advertiser : "$_id.advertiser", networks : {$size : "$networks"} , advertisers : {$size : "$publishers"} } 
}
]);

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这里非常详细地描述了这个确切的情况：嗨，皮特，但我想要两个数组计数。我有一系列的广告商，我想在上面运行这个