Question

我正在编写MongoDB应用程序，并且我在C＃中使用聚合管道进行复杂查询。当我在shell中复制C＃生成的聚合时，一切似乎都很好。但是，在C＃中执行聚合时，某些属性将设置为null。请参阅以下内容以获取更多信息。

首先，让我告诉你我的模特：

public class Smartphone
{
    #region Properties

    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }

    [BsonElement("name")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Name { get; set; }

    [BsonElement("description")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Description { get; set; }

    [BsonElement("typenr")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Type { get; set; }

    [BsonElement("props")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public List<SmartphoneProperty> Properties { get; set; }

    #endregion
}

public class UnwindedSmartphone
{
    #region Properties

    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }

    [BsonElement("name")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Name { get; set; }

    [BsonElement("description")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Description { get; set; }

    [BsonElement("typenr")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Type { get; set; }

    [BsonElement("props")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public SmartphoneProperty Property { get; set; }

    #endregion
}

public class SmartphoneProperty
{
    #region Properties

    [BsonElement("type")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Type { get; set; }

    [BsonElement("value")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string Value { get; set; }

    #endregion
}

我的收藏中的单个文档看起来如下所示：

{
    "_id" : ObjectId("55d45c0285afc59c146f66f0"),
    "name" : "LG Nexus 6",
    "description" : "A Nexus 6 device, created by Google.",
    "typenr" : "LG-NEX-5/BLACK",
    "props" : [ 
        {
            "type" : "os",
            "value" : "Android"
        }, 
        {
            "type" : "storage",
            "value" : "8"
        }, 
        {
            "type" : "storage",
            "value" : "16"
        }, 
        {
            "type" : "storage",
            "value" : "32"
        }, 
        {
            "type" : "storage",
            "value" : "64"
        }
    ]
}

需要执行的聚合命令如下：

// Get all the amount of filters that are defined.
db.smartphones.aggregate([
    // Unwind the "props".
    { "$unwind" : "$props" },

    // Grouping phase.
    // Group by unique properties, add a count for the amount of articles, and add articles to an element named "articles".
    // We use the function "$addToSet" here to ensure that only unique articles are being added.
    { 
        "$group" : { 
            "_id" : "$props", 
            count : { "$sum" : 1 }, 
            articles: { 
                "$addToSet": { 
                    name: "$name", 
                    description: "$description", 
                    typenr: "$typenr" 
                } 
            } 
        } 
    },

    // Sort the results based on the "_id" field.
    { "$sort" : { "_id" : 1 } }
]);

此聚合的单个结果将返回以下内容：

"_id" : {
    "type" : "storage",
    "value" : "128"
},
"count" : 1.0000000000000000,
"articles" : [ 
    {
        "name" : "Apple iPhone 6",
        "description" : "An iPhone 6 device, created by Apple.",
        "typenr" : "APP-IPHONE-6/BLACK"
    }
]

现在，在C＃中，我已按以下方式编写聚合：

var aggregation = collection.Aggregate()
    .Unwind<Smartphone, UnwindedSmartphone>(x => x.Properties)
    .Group(key => key.Property, g => new
    {
        Id = g.Key,
        count = g.Count(),
        articles = g.Select(x => new
        {
            name = x.Name,
            description = x.Description,
            typenr = x.Type
        }).Distinct()
    })
    .SortBy(x => x.Id);

如果我检查此聚合转换为的命令，则显示如下：

// Get all the amount of filters that are defined.
db.smartphones.aggregate([
    // Unwind the "props".
    { "$unwind" : "$props" }, 

    // Grouping phase.
    // Group by unique properties, add a count for the amount of articles, and add articles to an element named "articles".
    // We use the function "$addToSet" here to ensure that only unique articles are being added.
    { 
        "$group" : { 
            "_id" : "$props", 
            "count" : { "$sum" : 1 }, 
            "articles" : { 
                "$addToSet" : { 
                    "name" : "$name", 
                    "description" : "$description", 
                    "typenr" : "$typenr" 
                } 
            } 
        } 
    }, 
    { "$sort" : { "_id" : 1 } }
])

因此，它与我尝试转换为C＃代码的原始聚合相同，除了属性用双引号括起来，但这不是问题。

如果我在shell中执行此聚合，结果很好。但是，当它在C＃中执行时，articles属性的null，name和description的值为typenr。

任何知道为什么会这样的人？

Answer 1

确定，

感谢@BlakesSeven指出了我正确的方向（见他对我的问题的评论，我现在知道为什么我会遇到这个问题）。

我需要转换select中的group语句可能不会返回匿名类型。而不是那样，我需要返回一个打字对象。

这意味着我的代码需要更改为以下内容：

var aggregation = collection.Aggregate()
    .Unwind<Smartphone, UnwindedSmartphone>(x => x.Properties)
    .Group(key => key.Property, g => new
    {
        Count = g.Count(),
        Articles = g.Select(x => new AggregatedSmartphoneArticle
        {
            Name = x.Name,
            Description = x.Description,
            Type = x.Type
        }).Distinct()
    });

现在一切正常。再次感谢。使用类型化代码比使用BsonDocument更清晰。

对于使用MongoDB C＃Driver 2.0的某些属性，C＃中的聚合确实返回null

1 个答案: