MongoDB中嵌套数组的$ reduce和$ setUnion [英] $reduce and $setUnion for nested arrays in MongoDB

问题描述

我正在MongoDB中使用$ reduce和$ setUnion合并嵌套数组.以下是示例输入-

I am looking to merge nested arrays using $reduce and $setUnion in MongoDB. Following is the sample input -

levels: [
 [[80,100,120]],[[100,150]],[[200,80,100]],[[80,100]]
]

我打算得到的输出(即，将嵌套数组与唯一值合并)- 级别:[80,100,120,150,100]

The output what I am intended to get (i.e. merge nested arrays with unique values) - levels: [80,100,120,150,100]

我可以通过两种方法获得上述输出-

I am able to get the above output using two ways -

1)在管道中使用两个$ project阶段-

1) Using two $project stages in pipeline -

aggregate([
$project: { levels : { $reduce: {
input: "$levels", initialValue: [], in:{$setUnion:["$$value","$$this"]}}}},
$project: { levels : { $reduce: {
input: "$levels", initialValue: [], in:{$setUnion:["$$value","$$this"]}}}}
])

2)首先$ unwind，然后$ project

2) First $unwind and then $project

aggregate([
$unwind: { '$levels'}},
$project: { levels : { $reduce: {
input: "$levels", initialValue: [], in:{$setUnion:["$$value","$$this"]}}}}
])

在我不需要两次使用相同的$ project的情况下，有没有一种改进的第一选择方式?我正在努力避免放松.

Can there be any improved way of first option where I don't need to use the same $project twice? I am trying to avoid unwind.

推荐答案

您可以将以下聚合与单个 $project 阶段

You can use below aggregation with single $project stage

db.collection.aggregate([
  { "$project": {
    "levels": {
      "$reduce": {
        "input": {
          "$reduce": {
            "input": "$levels",
            "initialValue": [],
            "in": { "$setUnion": ["$$this", "$$value"] }
          }
        },
        "initialValue": [],
        "in": { "$setUnion": ["$$this", "$$value"] }
      }
    }
  }}
])

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