带有条件的 MongoDB 项目数组文档大小 [英] MongoDB Project Array Document Size with Condition

问题描述

假设我有以下文档结构:

Let's say I have the following document structure:

{
  "_id": "ID",
  "array": [
    {
      "a": "A",
      "b": "B",
      "c": {
        "x": true,
        "y": true,
        "z": false
      }
    },
    {
      "a": "A",
      "b": "B"
    },
    {
      "a": "A",
      "b": "B"
      "c": {
        "s": true
      }
    }
  ]
}

我正在尝试进行聚合以提供这种类型的输出:

I am trying to do an aggregation which gives me this type of output:

{
  "_id": "ID",
  "array": [
    {
      "a": "A",
      "b": "B",
      "c": 2
    },
    {
      "a": "A",
      "b": "B",
      "c": 0
    },
    {
      "a": "A",
      "b": "B"
      "c": 1
    }
  ]
}

所以我想要做的是，而不是文档 c，返回以 true 作为值的元素的数量.和示例一样，c 字段不一定存在(在这种情况下我想返回 0)，当它存在时，它的子字段不一定与其他 c 数组中的子字段.假设我正在进行聚合:

So what I want to do is to, instead of the document c, return the number of elements with true as value. And as in the example, the c field does not necessarily exist (in which case I want to return 0), and when it does, it's subfields are not necessarily the same as other c subfields in the array. Let's say I am doing the aggregation:

db.collection.aggregate([
  {"$match": {<conditions>}},
  {"$project": {
    "array.a": 1,
    "array.b": 1,
    "array.c": <?>,
  }}
])

如何在投影中调整 "array.c" 以完成我所追求的?

How can I adjust "array.c" in the projection to accomplish what I'm after?

推荐答案

至少需要有MongoDB v3.4.4

You need to have at least MongoDB v3.4.4

投影 "array.a": 1 将返回 ["A", "A", "A"].由于您的 array 字段是一个数组，我们需要使用 $map 运算符进行迭代.

Projecting "array.a": 1 will return ["A", "A", "A"]. Since your array field is an array, we need to use $map operator for iteration.

要遍历对象键，我们需要使用 将其转换为数组$objectToArray 运算符.

To iterate over object keys, we need to transform it into array with $objectToArray operator.

"c": {                   "c": [
    "x": true,             {k: "x", v: true},
    "y": true,       ->    {k: "y", v: true},
    "z": false             {k: "z", v: false},
  }                      ]

然后，我们应用 $filter 运算符来获取 v 为 true 的 k:v 对.

Then, we apply $filter operator to get just the k:v pairs whose v is true.

db.collection.aggregate([
  {
    $match: {}
  },
  {
    $project: {
      array: {
        $map: {
          input: "$array",
          as: "arr",
          in: {
            a: "$$arr.a",
            b: "$$arr.b",
            c: {
              $size: {
                $filter: {
                  input: {
                    $objectToArray: {
                      $ifNull: [
                        "$$arr.c",
                        {}
                      ]
                    }
                  },
                  cond: {
                    $eq: [
                      "$$this.v",
                      true
                    ]
                  }
                }
              }
            }
          }
        }
      }
    }
  }
])

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