如何求和mongodb檔案中陣列內所有物件中存在的特定陣列欄位的長度

這是我的mongodb檔案示例（嘗試使用jsonformatter.com進行分析）：

{"_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}

我想對 score 陣列內所有物件的 catches 陣列欄位的長度求和。我知道，我可以使用聚合框架來實作它，但我是 mongodb 的初學者，并且不了解許多聚合運算子。這是我嘗試過的聚合管道，但它回傳此欄位的存在數量，而不是此陣列的長度總和：

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

uj5u.com熱心網友回復：

1. $unwind- 將score陣列欄位解構為多個檔案。
2. $group- 分組依據null（對于所有物件），接下來$sum是$sizeof score.catches。

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

示例 Mongo Playground

注意：如果您希望結果基于每個檔案（而不是組合所有檔案），那么您需要將$group_id更改為：

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

標籤：MongoDB mongodb-查询 mongodb-指南针

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