MongoDB的理解和总结GROUPBY [英] Understanding MongoDB Aggregate and GroupBy

问题描述

我想通过ID做MongoDB中查询到第一组，然后降序排列。我有一个功能LINQ表达式在这里：

I'm trying to do a query in MongoDB to first group by id and then sort descending. I have a functional LINQ expression here:

var list = this.GetPackages().ToList();
      list = list.OrderByDescending(package => package.PackageVersion)
        .GroupBy(g => g.PackageId)
        .Select(packages => packages.First()).ToList();
      return list;

但我似乎无法拿出相当于MongoDB的表情，我也不能连得$项目功能工作：

But I can't seem to come up with the equivalent MongoDB expression, nor can I even get the $project function to work:

db.packages.aggregate([
    {
        $sort : { packageVersion : -1 }
    },
    {
        $group: { _id: "$PackageId" }
    },
    {
        $project: { PackageVersion: 1, Title: 1 }
    }
])

我的结果是这样的：

{ "_id" : "e3afb1fe-dce7-476e-8372-cd8201abc131" }
{ "_id" : "e3722179-0903-4894-9a86-3a3ffd94de83" }
{ "_id" : "3e65e93a-4c2c-4a02-8b21-e5858a4058dd" }

格式正确的MongoDB的查询，是有一个相当于办法做到这一点使用C＃MongoDB的驱动程序？

Is the MongoDB query of the correct format, and is there an equivalent way to do this using the C# MongoDB driver?

推荐答案

请使用 $第一个运算符和的 $$ ROOT 变量获得该组中的第一个文档。

Make use of the $first operator and $$ROOT variable to get the first document in the group.

$$ ROOT 是一个系统变量：

参考根文档，即顶级文档，目前$在聚合流水线级被处理b $湾

References the root document, i.e. the top-level document, currently being processed in the aggregation pipeline stage.

然后项目中第一个文件。

Then project the first document.

db.packages.aggregate([
    {
        $sort : { packageVersion : -1 }
    },
    {
        $group: { "_id": "$PackageId","firstPackage":{$first:"$$ROOT"}}
    },
    {
        $project: { "firstPackage": 1, "_id": 0}
    }
])

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