如何仅使用C＃从多层嵌入式MongoDB文档中获取具有相应父元素的确切子元素

Question

[尝试使用C＃从嵌入式MongoDB文档中获取确切的子文档及其对应的父文档，但查询返回所有子文档以及对应的父文档和其他文档。如何将参数设置为使用Filter和findOptions获得完全匹配。我的预期结果将是频道1->第1集，并且是给定ID的Child Track。这是我的代码：

// Class
namespace CrudWithMultilvelNestedDoc
{
    public class Channel
    {
        [BsonId]
        [BsonRepresentation(BsonType.String)]
        public string Id { get; set; }
        public string Name { get; set; }
        public Episode[] Episodes { get; set; }
    }

    public class Episode
    {
        [BsonId]
        [BsonRepresentation(BsonType.String)]
        public string Id { get; set; }
        public string Name { get; set; }
        public Track[] Tracks { get; set; }
    }

    public class Track
    {
        [BsonId]
        [BsonRepresentation(BsonType.String)]
        public string Id { get; set; }
        public string Name { get; set; }
        public Like[] Likes { get; set; }
    }

    public class Like
    {
        [BsonId]
        [BsonRepresentation(BsonType.String)]
        public string Id { get; set; }
        public string Name { get; set; }
    }
}

//Code
var chanelId = "5e4606e6ae7b090688671416";
var episodeId = "5e46071d385a672b0cea0f86";
var trackId = "5e460dbe2bc5e70c9cfeac21";

var filter = Builders<Channel>.Filter.And(
             Builders<Channel>.Filter.Where(x => x.Id == chanelId),
             Builders<Channel>.Filter.Eq("Episodes._id", episodeId),
             Builders<Channel>.Filter.Eq("Episodes.Tracks._id", trackId));
var result = collection.Find(filter).ToList();

// Data
{"_id":"5e4606e6ae7b090688671416","Name":"Channel 1","Episodes":[{"_id":"5e46071d385a672b0cea0f86","Name":"Episode 1","Tracks":[{"_id":"5e460dbe2bc5e70c9cfeac21","Name":"Trak 1","Likes":[{"_id":"5e4635d41f949d43a8194790","Name":"Like 1"},{"_id":"5e464e1ccfa17a25181234f6","Name":"Like 2"},{"_id":"5e464e94776dc808383326d5","Name":"Like 3"}]},{"_id":"5e4612d60747a2121870c815","Name":"Trak 2","Likes":[]}]},{"_id":"5e460851d29c1b3df4d27b7d","Name":"Episode 2","Tracks":[{"_id":"5e460e307ca6843758ce814e","Name":"Trak 1","Likes":[{"_id":"5e4686aa2813d41fa428b621","Name":"Like 1"}]}]}]}

Answer 1

这是mickl的答案的lambda版本，并且保留了频道信息：

var result = collection.AsQueryable()
               .Where(c => c.Id == "5e4606e6ae7b090688671416")
               .SelectMany(c => c.Episodes,
                           (c, e) => new
                           {
                               cid = c.Id,
                               cname = c.Name,
                               eid = e.Id,
                               ename = e.Name,
                               etracks = (Track[])e.Tracks.Where(t => t.Id == "5e460dbe2bc5e70c9cfeac21")
                           })
               .Where(x => x.etracks.Any())
               .Select(x => new Channel
               {
                   ID = x.cid,
                   Name = x.cname,
                   Episodes = new Episode[] {
                       new Episode {
                           Id = x.eid,
                           Name = x.ename,
                           Tracks = x.etracks } }
               })
               .ToList();

Answer 2

由于您的文档是多层的，因此必须使用$filter运算符。在C＃中，使用LINQ进行此类查询很容易，因为它会在后台自动生成$filter，并且您保留了强类型查询：

var q = from channel in mongoDBCollection.AsQueryable()
        from episode in channel.Episodes
        select new Episode()
        {
            Id = episode.Id,
            Name = episode.Name,
            Tracks = episode.Tracks.Where(x => x.Id == trackId)
        };

var query = q.Where(x => x.Tracks.Any());
var result = query.ToList();

由MongoDB .NET驱动程序生成的聚合管道如下所示：

{ 
    "aggregate" : "col", 
    "pipeline" : [
        { "$unwind" : "$Episodes" }, 
        { 
            "$project" : { 
                "Id" : "$Episodes._id", 
                "Name" : "$Episodes.Name", 
                "Tracks" : { 
                    "$filter" : { 
                        "input" : "$Episodes.Tracks", 
                        "as" : "x", 
                        "cond" : { "$eq" : ["$$x._id", "5e460dbe2bc5e70c9cfeac21"] } 
                    } 
                }, 
                "_id" : 0 
            } 
        }, 
        { 
            "$match" : { 
                "Tracks" : { 
                    "$ne" : null, 
                    "$not" : { "$size" : 0 } 
                } 
            } 
        }], 
    "cursor" : { }, 
}

编辑：

为了提高上述查询的性能，您可以考虑通过添加channel语句在where级别进行过滤：

from channel in mongoDBCollection.AsQueryable()
where channel.Id == channelId

编辑：当您还想返回Channel时，它会变得有些棘手。我的建议是返回一个既包含Channel又包含Episode的异常类型，并将它们组合在内存中，因为它将允许您在数据库端运行大多数过滤并获得最佳性能：

var q = from channel in mongoDBCollection.AsQueryable()
        where channel.Id == "5e4606e6ae7b090688671416"
        from episode in channel.Episodes
        select new 
        {
            Channel = new Channel(){ Id = channel.Id, Name = channel.Name},
            Episode = new Episode()
            {
                Id = episode.Id,
                Name = episode.Name,
                Tracks = episode.Tracks.Where(x => x.Id == trackId)
            }
        };

var query = q.Where(x => x.Episode.Tracks.Any());
var result = query.ToList();