如何对数组中项目的值进行分组和计数

Question

[
  {
    providers: [
      {
        code: 1
      },
      {
        code: 1
      },
    ]
  },
  {
    providers: [
      {
        code: 2
      },
      {
        code: 1
      },
      {
        code: 2
      }
    ]
  },
]

我有这样的文件

并希望获得如下结果：

[
  {
    "code": 1,
    "total": 2 (because it appear in two item of the array)
  },
  {
    "code": 2,
    "total": 1 (because it appear in just the second item of the array)
  }
]

我尝试使用 $sum: 1 展开数组和分组，但是当展开时，总数会增加，这不是我想要的。请帮我。

Answer 1

你有一个好主意，你只需要将你的 code 数组转换为 $setIntersection 设置。

见 mongoplayground https://mongoplayground.net/p/NwVcO9iBXAf

db.collection.aggregate([
  {
    $set: {
      providers: {
        "$setIntersection": "$providers"
      }
    }
  },
  {
    "$unwind": "$providers"
  },
  {
    "$group": {
      "_id": "$providers.code",
      "count": {
        "$sum": 1
      }
    }
  },
  {
    $project: {
      _id: 0,
      code: "$_id",
      total: "$count"
    }
  }
])