Mongodb求和数组字段的大小

问题描述

所以我有一堆简单的文档，例如

so i have a bunch of simple documents like

{

  "foos": [
    ObjectId("5105862f2b5e30877c685c58"),
    ObjectId("5105862f2b5e30877c685c57"),
    ObjectId("5105862f2b5e30877c685c56"),
  ],

  "typ": "Organisation",

}

，我想找出与组织"类型的文档相关的foos的总体大小

and i want to find out the overall size of associated foos to documents of type "Organisation"

所以我有这个汇总查询

db.profil.aggregate(
   [
   {
    $match:{
     "typ":"Organisation"
    }
   },
      {
         $project: {
            fooos: { $size: "$foos" }
         }
      }
   ]
)

这将返回每个文档的所有foo的计数

this returns the count of all foos for each document

like:

{ "_id" : ObjectId("50e577602b5e05e74b38a6c8"), "foooos" : 1 }
{ "_id" : ObjectId("51922170975a09f363e3eef5"), "foooos" : 3 }
{ "_id" : ObjectId("51922170975a09f363e3eef8"), "foooos" : 2 }
{ "_id" : ObjectId("5175441d975ae346a3a8dff2"), "foooos" : 0 }
{ "_id" : ObjectId("5192216f975a09f363e3eee9"), "foooos" : 2 }
{ "_id" : ObjectId("5192216f975a09f363e3eeeb"), "foooos" : 3 }
{ "_id" : ObjectId("5192216f975a09f363e3eee4"), "foooos" : 2 }
{ "_id" : ObjectId("5192216f975a09f363e3eee6"), "foooos" : 2 }
{ "_id" : ObjectId("5192216f975a09f363e3eedb"), "foooos" : 2 }
{ "_id" : ObjectId("51922174975a09f363e3ef4a"), "foooos" : 1 }
{ "_id" : ObjectId("5192216f975a09f363e3eee1"), "foooos" : 1 }
{ "_id" : ObjectId("5192216e975a09f363e3eed7"), "foooos" : 2 }
{ "_id" : ObjectId("5192216f975a09f363e3eeee"), "foooos" : 3 }

是否有一些查询可以返回所有文档的foo的总计数?

is there some query that would return the summed up count for foos of all documents ?

我用$ sum玩过，但是不知道如何与查询结合，我只会得到语法错误，知道这是否可行

i played arround with $sum but dont know how to combine with my query, i only do get syntax errors, it would be cool to know if this is possible

推荐答案

包括 $group 操作员流水线阶段，位于 $project 步骤如下:

Include the $group operator pipeline stage after the $project step as follows:

db.profil.aggregate([
   { "$match":{ "typ": "Organisation" } },
   { "$project": {
         "fooos": { "$size": "$foos" }
   } },
   { "$group": {
       "_id": null,
       "count": {
           "$sum": "$fooos"
       }
   } }
])

这会将先前 $project 阶段并应用累加器表达式fooos字段上的="noreferrer"> $sum 以获取总计(使用上一个示例):

This will group all the input documents from the previous $project stage and applies the accumulator expression $sum on the fooos field within the group to get the total (using your last example):

这也可以通过 $project 管道为:

This can also be done by-passing the $project pipeline as:

db.profil.aggregate([
   { "$match": { "typ": "Organisation" } },
   { "$group": {
       "_id": null,
        "count": {
            "$sum": { "$size": "$foos" }
        }
    } }
])

输出

/* 0 */
{
    "result" : [
        {
            "_id" : null,
            "count" : 24
        }
    ],
    "ok" : 1
}

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