从多对多关系中检索对象

Question

我正在使用 Entity Framework Code First，我的模型中有以下对象：

public class Category {
    [DatabaseGenerated( DatabaseGeneratedOption.Identity ), Key]
    public int CategoryId { get; set; }

    [Required, MaxLength( 128 ), Index( IsUnique = true)]
    public string CategoryName { get; set; }

    public string Description { get; set; }

    public virtual ICollection<Article> Articles { get; set; }
}

public class Article {
    [DatabaseGenerated( DatabaseGeneratedOption.Identity ), Key]
    public int ArticleId { get; set; }

    [Required, MaxLength( 128 ), Index]
    public string ArticleName { get; set; }

    public string Description { get; set; }

    public virtual ICollection<Category> Categories { get; set; }
}

我正在我的数据访问层中实现一种方法来检索所选 Categories:

之一中的所有 Articles

IEnumerable<Article> GetArticles( int[] categoryIds );

我的问题：如何构建查询表达式？如果我在 SQL 中执行此操作，我会编写如下查询：

SELECT a.*
FROM Articles a
JOIN ArticlesInCategories AS aic ON a.ArticleId = aic.ArticleId
JOIN Categories AS c on aic.CategoryId = c.CategoryId
WHERE c.CategoryId IN ( . . . )

那是否意味着我会这样写？

return ( from a in context.Articles
         join c in context.Categories ON a.CategoryId = c.CatgegoryId
         where categoryIds.Contains( c.CategoryId ) );

唯一的问题是 Article class 没有 CategoryId 属性.

如何构建此查询？

Answer 1

您需要反转查询并使用 SelectMany():

return ( from c in context.Categories
         where categoryIds.Contains( c.CategoryId ) )
        .SelectMany(category => category.Arcticles);

从多对多关系中检索对象

Retrieve objects from many to many relationship

c#

many-to-many

entity-framework

entity-framework-6