如何使用MongoDB C#驱动程序2.0创建流畅的聚合操作

Question

我对MongoDB还比较新，正在将其用于Web Api以为移动应用提供服务。

现在，我需要运行聚合操作，并且由于我使用C#，因此希望通过在集合上使用Aggregate命令来流畅地执行它，从而返回IAggregateFluent。

但是，我遇到了问题，并且在这里找到的信息并没有帮助我，因此提出了新的问题。

我已经构建了一个小型集合，其中包含一些基本属性的智能手机，并且智能手机集合中的单个项如下：

{
    "name" : "LG Nexus 5",
    "description" : "A Nexus 5 device, created by Google.",
    "typenr" : "LG-NEX-5/WHITE",
    "props" : [ 
        {
            "type" : "os",
            "value" : "Android"
        }, 
        {
            "type" : "storage",
            "value" : "8"
        }, 
        {
            "type" : "storage",
            "value" : "16"
        }, 
        {
            "type" : "storage",
            "value" : "32"
        }, 
        {
            "type" : "storage",
            "value" : "64"
        }
    ]
}

现在，我已经在shell中创建了一个聚合，它看起来像下面这样：

// 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" 
                } x =>
            } 
        } 
    },

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

现在我需要将这个内容翻译为C#代码。

首先，我创建了以下代码（纯C#代码，只返回一个IMongoCollection<Article>）。

var collection = context.ArticleRepository;

这是集合返回的模型：

public class Article
{
    #region Properties

    /// <summary>
    ///     Unique identifier for the article.
    /// </summary>
    [BsonRepresentation(BsonType.ObjectId)]
    public string Id { get; set; }

    /// <summary>
    ///     Name of the article.
    /// </summary>
    [BsonElement("name")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public BsonString Name { get; set; }

    /// <summary>
    ///     Name of the element but in lowercase.
    /// </summary>
    /// <remarks>
    ///     We'll create this field to enable text-search on this field without respecting capital letters.
    /// </remarks>
    [BsonElement("namelc")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public BsonString LowercaseName { get; set; }

    /// <summary>
    ///     Specification of the article.
    /// </summary>
    [BsonElement("specification")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public BsonString Specificiation { get; set; }

    /// <summary>
    ///     Brand of the article.
    /// </summary>
    [BsonElement("brand")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public BsonString Brand { get; set; }

    /// <summary>
    ///     Supplier of the article.
    /// </summary>
    [BsonElement("supplier")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public Supplier Supplier { get; set; }

    /// <summary>
    ///     Number of the article.
    /// </summary>
    [BsonElement("nr")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public BsonString ArticleNumber { get; set; }

    /// <summary>
    ///     Gtin number of the article.
    /// </summary>
    [BsonElement("gtin")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string ArticleGtin { get; set; }

    /// <summary>
    ///     type number of the article.
    /// </summary>
    [BsonElement("typeNr")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public string TypeNumber { get; set; }

    /// <summary>
    ///     Properties of the article.
    /// </summary>
    /// <remarks>
    ///     This field can be used to ensure that we can filter on the articles.
    ///     By default, this is an empty list, this avoids initialization logic.
    /// </remarks>
    [BsonElement("props")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public List<ArticleProperty> Properties { get; set; } = new List<ArticleProperty>();

    #endregion
}

/// <summary>
///     Class representing a single supplier in the database.
/// </summary>
/// <remarks>
///     This class is not used as a "root" document inside our database.
///     Instead, it's being embedded into our "Articles" document.
/// </remarks>
public class Supplier
{
    #region Properties

    /// <summary>
    ///     Name of the supplier.
    /// </summary>
    [BsonElement("supplier")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public BsonString Name { get; set; }

    /// <summary>
    ///     Gln of the supplier.
    /// </summary>
    [BsonElement("gln")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public BsonString Gln { get; set; }

    #endregion
}

/// <summary>
///     Class representing a single property for an article in the database.
/// </summary>
/// <remarks>
///     This class is not used as a "root" document inside our database.
///     Instead, it's being embedded into our "Articles" document.
/// </remarks>
public class ArticleProperty
{
    #region Properties

    /// <summary>
    ///     Type of the property.
    /// </summary>
    [BsonElement("type")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public BsonString Type { get; set; }

    /// <summary>
    ///     Value of the property.
    /// </summary>
    [BsonElement("value")]
    [BsonIgnoreIfNull]
    [BsonIgnoreIfDefault]
    public BsonString Value { get; set; }

    #endregion
}

现在，我需要对这个集合进行聚合操作，但已经在基础方面遇到了困难：

// Build the aggregation using the fluent api.
var aggregation = collection.Aggregate()
    .Unwind(x => x.Properties)
    .Group(x => new { x.Properties );

目前，我只尝试按属性进行分组，就像聚合一样，但这会导致以下错误：

CS0411 方法“IAggregateFluent<BsonDocument>.Group<TNewResult>(ProjectionDefinition<BsonDocument, TNewResult>)”的类型参数无法从使用中推断出。请尝试明确指定类型参数。

即使这样做可以工作，我还需要额外的属性，如count和addToSet。有人能帮我解决吗？我已经寻找了两天，这让我发疯了。

编辑

我发现在C#中，跟随unwind的group有效，但为什么先使用unwind就不起作用了呢？我真的需要先执行unwind。

编辑2 我设法使一小部分有效，包括group命令。请参见以下代码：

var aggregation = collection.Aggregate()
    .Unwind<Smartphone, UnwindedSmartphone>(x => x.Properties)
    .Group(key => key.Property, g => new
    {
        Id = g.Key,
        Count = g.Count()
    });

然而，我需要更多关于如何从聚合命令中推送文章属性的信息。

- Complexity

Answer 1

我已经找到了解决问题的方法。 应该使用以下C#代码：

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
        }).Distinct()
    })
    .SortBy(x => x.Id);

这给我以下聚合结果：

db.smartphones.aggregate([{ "$unwind" : "$props" }, { "$group" : { "_id" :     "$props", "Count" : { "$sum" : 1 }, "Articles" : { "$addToSet" : { "Name" :     "$name" } } } }, { "$sort" : { "_id" : 1 } }])

- Complexity