Mongodb聚合:$ reduce不能按预期工作 [英] Mongodb aggregation: $reduce not working as expected

问题描述

我有一个mongodb聚合 $ reduce pipleine无法正常工作.这是我要实现的目标.

I have a mongodb aggregation $reduce pipleine that is not working as expected. This is what I am trying to achieve.

基本上，我试图在给定属性中获取具有最高值的对象.在某些对象中， $ reduce 返回错误的对象，而在其他对象中，返回 null ，这意味着没有对象满足条件.

Basically I am trying to get the object with the highest value in a given property. In some objects $reduce returns the wrong object in others it returns null, meaning no object satisfied the condition.

我的代码具有group阶段和其他阶段，这些阶段会生成在 $ reduce 阶段中使用的变量.聚合管道中是否有任何已知的先前阶段可能会影响 $ reduce 阶段?

My code has the group stage and other stage that produce the variable used in the $reduce stage. Are there any known preceding stages in the aggregation pipeline that might be affecting the $reduce stage?

推荐答案

• $ max 从字段 a 的 key 数组中获取最大值，这将返回 -15 作为根据您的文件
• $ filter 获取等于 -15 值的对象
• $ first 从 $ filter
返回的结果中获取第一个对象
• $max to get max value from key array of field a, this will return -15 as per your documents
• $filter to get object that equal to -15 value
• $first get first object from returned result from $filter

db.collection.aggregate([
  {
    $addFields: {
      winner: {
        $first: {
          $filter: {
            input: "$key",
            cond: { $eq: ["$$this.a", { $max: "$key.a" }] }
          }
        }
      }
    }
  }
])

游乐场

使用 $ reduce 运算符的第二个选项

Second option using $reduce operator,

• 在字段 a
的 key 数组中的reduce，最大值中设置初始字段 maxValue
• 检查条件，如果 maxValue 和 a 值匹配，则返回最大对象
• set initial field maxValue in reduce, maximum value from key array of field a
• check condition if maxValue and a value match then return max object

db.collection.aggregate([
  {
    $addFields: {
      winner: {
        $reduce: {
          input: "$key",
          initialValue: { maxValue: { $max: "$key.a" } },
          in: {
            $cond: [
              { $eq: ["$$this.a", "$$value.maxValue"] },
              "$$this",
              "$$value"
            ]
          }
        }
      }
    }
  }
])

游乐场

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