如何使用 mongodb C# 驱动程序获取子文档的集合？答案

【问题标题】：How to fetch collection of subdocument using mongodb C# driver?如何使用 mongodb C# 驱动程序获取子文档的集合？
【发布时间】：2021-01-14 06:35:41
【问题描述】：

我有以下 MongoDB 文档：

{
  "_id": {
    "$oid": "5fbfa0005c15aaf2eac69ba6"
  },
  "PostMedia": [
    {
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img1.jpg",
      "Title": null,
      "Description": ""
    },
    {
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img2.jpg",
      "Title": null,
      "Description": ""
    }
  ]
},
{
  "_id": {
    "$oid": "5fbfa0485c15aaf2eac69ba7"
  },
  "PostMedia": [
    {
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img3.jpg",
      "Title": null,
      "Description": ""
    },
    {
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img4.jpg",
      "Title": null,
      "Description": ""
    }
  ]
}

我想使用 MongoDB C# 驱动程序将所有 PostMedia 获取到单个数组。这是预期的结果：

[
    {
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img1.jpg",
      "Title": null,
      "Description": ""
    },
    {
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img2.jpg",
      "Title": null,
      "Description": ""
    }, {
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img3.jpg",
      "Title": null,
      "Description": ""
    },
    {
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img4.jpg",
      "Title": null,
      "Description": ""
    }
  ]

我曾尝试使用组聚合函数，但它返回了一个数组数组。

我收到的结果：

PostMedia:[{
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img1.jpg",
      "Title": null,
      "Description": ""
    },
    {
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img2.jpg",
      "Title": null,
      "Description": ""
    }],[
     {
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img3.jpg",
      "Title": null,
      "Description": ""
    },
    {
      "FilePath": "http://localhost:8886/localhost44323/image/",
      "Filename": "img4.jpg",
      "Title": null,
      "Description": ""
    }
  ]

目前我写的C#代码如下：

  var group = new[]
                {
                new BsonDocument("$group",
                new BsonDocument
                {
                    { "_id", BsonNull.Value },
                    { "PostMedia", new BsonDocument("$push", "$PostMedia") }
                })
            };
                var result = collection.Aggregate<MediaList>(group).FirstOrDefault();
                return list;
            }

有没有办法通过合并它们来获取子文档是一个数组。

【问题讨论】：

标签： c# mongodb mongodb-query aggregation-framework mongodb-.net-driver

【解决方案1】：

你必须在$group之前$unwind，

$unwind解构PostMedia数组
$replaceRoot 将 PostMedia 对象替换为根
$unset 删除 _id 字段

  { $unwind: "$PostMedia" },
  {
    $group: {
      _id: null,
      PostMedia: { $push: "$PostMedia" }
    }
  },
  { $unset: "_id" } // remove field

Playground

C#：

new BsonArray
{
    new BsonDocument("$unwind", "$PostMedia"),
    new BsonDocument("$group", 
    new BsonDocument
        {
            { "_id", BsonNull.Value }, 
            { "PostMedia", 
    new BsonDocument("$push", "$PostMedia") }
        })
    new BsonDocument("$unset", "_id")
}

【讨论】：

有没有办法忽略_id
需要在$group stage后添加$unsetstage，{ $unset: "_id" }
{ $replaceRoot: { newRoot: "$PostMedia" } } 向我抛出错误这里有什么问题你能指导我吗
什么是错误，在哪里抛出错误？。我不知道 c# 错误是否来自 c#，但会尝试。
你能通过这个sn-p吗mongoplayground.net/p/w_hoDgvWmcq