MongoDB Compass 选择不同的字段

Question

我正在尝试在 MongoDB Compass 中为我的一个集合中的字段城市创建一个聚合管道。这是我目前所拥有的：

[{$unwind: {
  path: '$city',
  preserveNullAndEmptyArrays: true
}}, {$group: {
  _id: null,
  distinctCities: { $addToSet: '$city' }
}}]

如何添加每个城市的出现次数？我想要一个包含城市和计数的对象的数组。

Answer 1

查询：

db.collection.aggregate([
    /** 
     * unwind city, if your desired response is just an array `[{city :..., count :... }]`
     * then you don't need to `preserveNullAndEmptyArrays`
     */
    {
      $unwind: "$city"
    },
    /** group on unique city's & count no.of occurrences */
    {
      $group: { _id: "$city", count: { $sum: 1 } }
    },
    /** group on all docs (without any condition) & push objects to an array */
    {
      $group: { _id: "", citiesAndCount: { $push: { city: "$_id", count: "$count" } } }
    }
  ])

测试： mongoplayground

参考： aggregation-pipeline

Answer 2

这在 MongoDB 中运行良好：

 const aggregatorOptionsCity = [
    { $match: req.query },
    {
      $group: {
        _id: '$city',
        count: { $sum: 1 },
      },
    },
  ];