python - MongoDB 分组

Question

我正在尝试计算按性别分组的平均年龄。预期输出应该是

{'_id': {'gender': 'M'},{'avg': '27.1}}
{'_id': {'gender': 'F'}},{'avg': '29.0}}

这是我的代码

pipeline = [
    {
        "$project" : { 
            "_id" : {'gender':"$gender"},
            "avg": {"$avg":"$age"},
        }
    },
    {"$group" : {"_id":"$_id"}},

]

for doc in db.user.aggregate(pipeline):
    print(doc)

现在的输出是:

{'_id': {'gender': 'M'}}
{'_id': {'gender': 'F'}}

如有任何帮助，我们将不胜感激!我知道这可能很容易......

Answer 1

以下管道将为您提供每种性别的平均年龄。您需要在投影之前进行分组。

    [
        {
            "$group": {
                "_id": "$gender",
                "avrg": {
                    "$avg": "$age"
                }
            }
        },
        {
            "$project": {
                "gender": "$_id",
                "avg": "$avrg",
                "_id": 0
            }
        }
    ]

这是生成上述内容的 C# 测试程序:

using MongoDB.Entities;
using MongoDB.Entities.Core;
using System;
using System.Linq;

namespace StackOverflow
{
    public class Person : Entity
    {
        public string Name { get; set; }
        public string Gender { get; set; }
        public int Age { get; set; }
    }

    public class Program
    {
        private static void Main(string[] args)
        {
            new DB("test", "localhost");

            (new[] {
                new Person{ Name = "one", Gender = "M", Age = 30},
                new Person{ Name = "two", Gender = "M", Age = 30},
                new Person{ Name = "three", Gender = "F", Age = 40},
                new Person{ Name = "four", Gender = "F", Age = 40},
            }).Save();

            var res = DB.Queryable<Person>()
                        .GroupBy(p => p.Gender)
                        .Select(g => new
                        {
                            gender = g.Key,
                            avg = g.Average(p => p.Age)
                        })
                        .ToArray();

            foreach (var r in res)
            {
                Console.WriteLine($"Gender: {r.gender} Average: {r.avg}");
            }

            Console.ReadKey();
        }
    }
}