如何对 mongodb 文档中数组内所有对象中存在的特定数组字段的长度求和

Question

这是我的mongodb文档的样本（尝试用jsonformatter.com分析一下）：

{"_id":"6278686","playerName":"Rohit Lal","tournamentId":"197831","score":[{"_id":"1611380","runsScored":0,"ballFaced":0,"fours":0,"sixes":0,"strikeRate":0,"oversBowled":0,"runsConceded":0,"economyRate":0,"wickets":0,"maiden":0,"howToOut":"-","catches":["Mohit Mishra"],"stumping":[],"runout":[],"participatedRunout":[]},{"_id":"1602732","runsScored":0,"ballFaced":0,"fours":0,"sixes":0,"strikeRate":0,"oversBowled":0,"runsConceded":0,"economyRate":0,"wickets":0,"maiden":0,"howToOut":"-","catches":[],"stumping":[],"runout":[],"participatedRunout":[]},{"_id":"1536514","runsScored":1,"ballFaced":3,"fours":0,"sixes":0,"strikeRate":33.33,"oversBowled":0,"runsConceded":0,"economyRate":0,"wickets":0,"maiden":0,"howToOut":"run out Sameer Baveja","catches":[],"stumping":[],"runout":[],"participatedRunout":[]},{"_id":"1536474","runsScored":2,"ballFaced":7,"fours":0,"sixes":0,"strikeRate":28.57,"oversBowled":0,"runsConceded":0,"economyRate":0,"wickets":0,"maiden":0,"howToOut":"c Rajesh b Prasad Naik","catches":[],"stumping":[],"runout":[],"participatedRunout":[]},{"_id":"1536467","runsScored":0,"ballFaced":0,"fours":0,"sixes":0,"strikeRate":0,"oversBowled":0,"runsConceded":0,"economyRate":0,"wickets":0,"maiden":0,"howToOut":"-","catches":[],"stumping":[],"runout":[],"participatedRunout":[]},{"_id":"1500825","runsScored":0,"ballFaced":0,"fours":0,"sixes":0,"strikeRate":0,"oversBowled":0,"runsConceded":0,"economyRate":0,"wickets":0,"maiden":0,"howToOut":"-","catches":[],"stumping":[],"runout":[],"participatedRunout":[]},{"_id":"1461428","runsScored":18,"ballFaced":6,"fours":1,"sixes":2,"strikeRate":300,"oversBowled":0,"runsConceded":0,"economyRate":0,"wickets":0,"maiden":0,"howToOut":"not out","catches":[],"stumping":[],"runout":[],"participatedRunout":[]},{"_id":"1461408","runsScored":0,"ballFaced":1,"fours":0,"sixes":0,"strikeRate":0,"oversBowled":0,"runsConceded":0,"economyRate":0,"wickets":0,"maiden":0,"howToOut":"c Sudhir b Vinay Kasat *vk*","catches":[],"stumping":[],"runout":[],"participatedRunout":[]},{"_id":"1451175","runsScored":0,"ballFaced":0,"fours":0,"sixes":0,"strikeRate":0,"oversBowled":0,"runsConceded":0,"economyRate":0,"wickets":0,"maiden":0,"howToOut":"-","catches":[],"stumping":[],"runout":[],"participatedRunout":[]},{"_id":"1451146","runsScored":0,"ballFaced":0,"fours":0,"sixes":0,"strikeRate":0,"oversBowled":0,"runsConceded":0,"economyRate":0,"wickets":0,"maiden":0,"howToOut":"-","catches":[],"stumping":[],"runout":[],"participatedRunout":[]},{"_id":"1392796","runsScored":0,"ballFaced":1,"fours":0,"sixes":0,"strikeRate":0,"oversBowled":0,"runsConceded":0,"economyRate":0,"wickets":0,"maiden":0,"howToOut":"c †Vinay Kedia b Lalit","catches":[],"stumping":[],"runout":[],"participatedRunout":[]}],"__v":0}

我想对 score 数组中所有对象的 catches 数组字段的长度求和。我知道，我可以使用聚合框架来实现它，但我是 mongodb 的初学者并且不了解许多聚合运算符。这是我尝试过的聚合管道，但它 returns 这个字段的存在数量，而不是这个数组的长度总和：

[
    "totalCatches": {
        $size: "$score.catches"
     }
]

Answer 1

$unwind - 解构 score 数组字段到多个文档。
$group - 按 null 分组（对于所有对象），下一个 $sum 用于 score.catches 的 $size。

db.collection.aggregate([
  {
    $unwind: "$score"
  },
  {
    $group: {
      _id: null,
      "totalCatches": {
        $sum: {
          $size: "$score.catches"
        }
      }
    }
  }
])

Sample Mongo Playground

注意：如果你希望结果基于每个文档（而不是合并所有文档），那么你需要将$group的_id更改为:

{
    $group: {
      _id: "$_id",
      ...
   }   
}

如何对 mongodb 文档中数组内所有对象中存在的特定数组字段的长度求和

How to sum the length of particular array field present in all objects inside an array in mongodb document

mongodb

mongodb-query

mongodb-compass