Linq分組 - <k,v>我的'V'在哪裏？

Question

在下面的代碼：

var finalArticles = 
     from domainArticle in articlesFoundInDomain 

     join articleCategoryVersion in dbc.ArticlesCategoriesVersions 
      on domainArticle.ArticleID equals articleCategoryVersion.ArticleID 
     join articleCategory in dbc.ArticleCategories 
      on articleCategoryVersion.CategoryID equals articleCategory.CategoryID 

     where articleCategory.ParentID == 52 

     group articleCategory by articleCategory.CategoryID 
      into newArticleCategoryGroup 

據我所知，該組子句應該返回一個IEnumerable其中k是密鑰，在這種情況下類別ID。

我覺得我現在誤解了Linq，因爲我認爲每個'k'都應該有'v'中的文章列表，但我不明白這些機制或術語或其他。當我嘗試將此語句投影到一個新的匿名對象中時，我似乎沒有得到任何文章......他們在哪裏？

編輯： 好了，所以我有一塊正在工作的代碼，但不幸的是它的命中SQL服務器多次：

var articlesAssociatedWithKnowledgeTypes = 
       from categories in dbc.ArticleCategories 

       join categoryVersions in dbc.ArticlesCategoriesVersions 
        on categories.CategoryID equals categoryVersions.CategoryID 

       join articles in articlesFoundInGivenDomain 
        on categoryVersions.ArticleID equals articles.ArticleID 

       where categories.ParentID == 52 && articles.Version == categoryVersions.Version 

       select new 
       { 
        ArticleID = articles.ArticleID, 
        ArticleTitle = articles.Title, 
        ArticleVersion = articles.Version, 
        CategoryID = categories.CategoryID, 
        CategoryName = categories.Name 
       } into knowledgeTypesFlat 
       group knowledgeTypesFlat by new { knowledgeTypesFlat.CategoryID, knowledgeTypesFlat.CategoryName } into knowledgeTypesNested 
       select new 
       { 
        CategoryID = knowledgeTypesNested.Key.CategoryID, 
        CategoryName = knowledgeTypesNested.Key.CategoryName, 
        Articles = knowledgeTypesNested.ToList() 
       }; 

我認爲ToList（）上的文章將排序那出來，但它沒有。但是，代碼的工作原理雖然我不確定這是否是最優的？

Answer 1

該分組返回IGrouping的枚舉。 IGrouping<K, V>本身實現IEnumerable<V>。將每個組視爲該組中所有成員的可枚舉數加上一個額外的屬性Key

Answer 2

在您的第一個查詢中，您顯示的是group by，第二個是group join，兩者都返回不同的結果。該組通過返回IEnumerable<IGrouping<TKey, TElement>>。爲了得到結果你期待，你可以按CategoryId和CategoryName和項目作爲我在下面：

var finalArticles = 
    from domainArticle in articlesFoundInDomain 
    join articleCategoryVersion in dbc.ArticlesCategoriesVersions 
     on domainArticle.ArticleID equals articleCategoryVersion.ArticleID 
    join articleCategory in dbc.ArticleCategories 
     on articleCategoryVersion.CategoryID equals articleCategory.CategoryID 
    where articleCategory.ParentID == 52 
    group articleCategory by new{ articleCategory.CategoryID,articleCategory.CategoryName} 
     into g 
    select new {CatId=g.Key.CategoryID, CatName=g.Key.CategoryName,Articles =g.ToList() }; 

當你需要的分組元素可以撥打ToList或ToArray正如我上面

Answer 3

沒有你finalArticles查詢結果爲IEnumerable<IGrouping<int, Article>>（假設CategoryID爲int，您的文章類型爲Article）。

這些IGrouping<int, Article>提供int類型（的Key財產您CategoryID，也是IEnumerable<Article>代表的物品爲CategoryID序列。

你可以把這個例如到Dictionary<int, List<Article>>映射CategoryID s到的物品清單：

var dictionary = finalArticles.ToDictionary(group => group.Key, group => group.ToList()); 

或類別的列表包含文章：

您的評論後

var categories = finalArticles.Select(group => new { 
       CategoryID = group.Key, 
       Articles = group.ToList()}).ToList(); 

---

更新：

var finalArticles = 
     from domainArticle in articlesFoundInDomain 
     join articleCategoryVersion in dbc.ArticlesCategoriesVersions 
      on domainArticle.ArticleID equals articleCategoryVersion.ArticleID 
     join articleCategory in dbc.ArticleCategories 
      on articleCategoryVersion.CategoryID equals articleCategory.CategoryID 

     where articleCategory.ParentID == 52 

     group articleCategory by new {articleCategory.CategoryID, articleCategory.Name} 
     into newArticleCategoryGroup 
     select new 
     { 
      CategoryID = newArticleCategoryGroup.Key.CategoryID, 
      CategoryName = newArticleCategoryGroup.Key.Name, 
      Articles = newArticleCateGroup.ToList() 
     }