没有$ unwind的$ group内部数组值 [英] $group inner array values without $unwind

问题描述

我想按指定字段的相同值对数组中的对象进行分组，并产生一个计数.

I want to group objects in the array by same value for specified field and produce a count.

我有以下mongodb文档(不存在不相关的字段).

I have the following mongodb document (non-relevant fields are not present).

{
  arrayField: [ 
    { fieldA: value1, ...otherFields }, 
    { fieldA: value2, ...otherFields },
    { fieldA: value2, ...otherFields } 
  ],
  ...otherFields
}

以下是我想要的.

{
  arrayField: [ 
    { fieldA: value1, ...otherFields }, 
    { fieldA: value2, ...otherFields },
    { fieldA: value2, ...otherFields } 
  ],
  newArrayField: [ 
    { fieldA: value1, count: 1 }, 
    { fieldA: value2, count: 2 },
  ],
  ...otherFields
}

在这里，我按fieldA对嵌入式文档进行了分组.

Here I grouped embedded documents by fieldA.

我知道如何通过以下两种方式完成放松和2个小组练习.(忽略了无关的阶段)

I know how to do it with unwind and 2 group stages the following way. (irrelevant stages are ommited)

具体示例

// document structure
{
  _id: ObjectId(...),
  type: "test",
  results: [ 
    { choice: "a" }, 
    { choice: "b" },
    { choice: "a" } 
  ]
}

db.test.aggregate([
{ $match: {} },
{
  $unwind: {
    path: "$results",
    preserveNullAndEmptyArrays: true
  }
},
{
  $group: {
    _id: {
      _id: "$_id",
      type: "$type",
      choice: "$results.choice",
    },
    count: { $sum: 1 }
  }
},
{
  $group: {
    _id: {
      _id: "$_id._id",
      type: "$_id.type",
      result: "$results.choice",
    },
    groupedResults: { $push: { count: "$count", choice: "$_id.choice" } }
  }
}
])

推荐答案

您可以在下面的 <代码中使用>聚合

db.test.aggregate([
  { "$addFields": {
    "newArrayField": {
      "$map": {
        "input": { "$setUnion": ["$arrayField.fieldA"] },
        "as": "m",
        "in": {
          "fieldA": "$$m",
          "count": {
            "$size": {
              "$filter": {
                "input": "$arrayField",
                "as": "d",
                "cond": { "$eq": ["$$d.fieldA", "$$m"] }
              }
            }
          }
        }
      }
    }
  }}
])

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