保持多对多实体框架关系中的外键列表

Question

我的代码优先实体框架模型中有多对多的关系.想象一下,我们有两个表,"公司"和"文章",它们之间有这种关系.我的简化代码模型如下所示:

public class Article
{
    public int Id { get; set; }
    public string Text { get; set; }
    public virtual ICollection<Company> Companies { get; set; }
}

public class Company
{
    public int Id { get; set; }
    public string Name { get; set; }
    public virtual ICollection<Article> Articles { get; set; }
}

使用流畅的映射,我创建了多对多关系:

modelBuilder.Entity<Company>().HasMany(c => c.Articles).WithMany(a => a.Companies);

这很好用,EF创建中间表.但是,根据我的逻辑,将每个实体与外键集合在一起会非常好.这意味着我想将以下属性添加到相应的模型类:

public virtual ICollection<int> ArticlesId { get; set; } // to Company
public virtual ICollection<int> CompaniesId { get; set; } // to Article

我知道一个解决方案是在EF中创建中间表模型并在每次调用中手动选择适当的ID,但是EF映射可以提供更方便的方法来执行此类操作吗？提前感谢您的任何提示.

Answer 1

似乎很遗憾没有办法以我想要的方式映射ID.但是,以下是有关如何实现所需实体密钥检索的三种解决方法.

Ben Reich建议的第一个解决方案.
实现get-only属性,该属性仅返回链接实体的ID.

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

    public IEnumerable<int> ArticlesIds
    {
        get { return Articles.Select(a => a.Id); }
    }
}

它似乎很方便使用,但它有一个缺点 - 整个实体将从数据库中读取,以便接收唯一的ID.以下是来自SQL事件探查器的此类调用的日志:

exec sp_executesql N'SELECT 
[Extent2].[Id] AS [Id], 
[Extent2].[Header] AS [Header], 
[Extent2].[Description] AS [Description], 
[Extent2].[Text] AS [Text], 
[Extent2].[CreationDate] AS [CreationDate], 
[Extent2].[AccountId] AS [AccountId], 
[Extent2].[ImageSetId] AS [ImageSetId]
FROM  [dbo].[CompanyArticles] AS [Extent1]
INNER JOIN [dbo].[Articles] AS [Extent2] ON [Extent1].[Article_Id] = [Extent2].[Id]
WHERE [Extent1].[Company_Id] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=1

二解决方案.
使用相同的模型,在读取实体后单独读取ID.

var ids = db.Set<Article>().Where(a => a.Companies.Select(c => c.Id).Contains(f.Id)).ToList();

这种方法与前一种方法完全相同,将获取整个实体集.

exec sp_executesql N'SELECT 
[Extent1].[Id] AS [Id], 
[Extent1].[Header] AS [Header], 
[Extent1].[Description] AS [Description], 
[Extent1].[Text] AS [Text], 
[Extent1].[CreationDate] AS [CreationDate], 
[Extent1].[AccountId] AS [AccountId], 
[Extent1].[ImageSetId] AS [ImageSetId]
FROM [dbo].[Articles] AS [Extent1]
WHERE  EXISTS (SELECT 
    1 AS [C1]
    FROM [dbo].[ArticleCompanies] AS [Extent2]
    WHERE ([Extent1].[Id] = [Extent2].[Article_Id]) AND ([Extent2].[Company_Id] = @p__linq__0)
)',N'@p__linq__0 int',@p__linq__0=1

第三种方案.从我的角度来看,最合适的.
为中间表创建实体类.

public class ArticleCompany
{
    public int CompanyId { get; set; }
    public int ArticleId { get; set; }

    public virtual Company Company { get; set; }
    public virtual Article Article { get; set; }
}

使用此实体将两个实体映射为1对m关系.不要忘记映射新实体本身.

modelBuilder.Entity<Article>().HasMany(a => a.ArticlesCompanies).WithRequired(ac => ac.Article).HasForeignKey(ac => ac.ArticleId);
modelBuilder.Entity<Company>().HasMany(c => c.ArticlesCompanies).WithRequired(ac => ac.Company).HasForeignKey(ac => ac.CompanyId);

modelBuilder.Entity<ArticleCompany>().ToTable("ArticlesCompanies");
modelBuilder.Entity<ArticleCompany>().HasKey(ac => new { ac.ArticleId, ac.CompanyId });

然后,在获取实体后,使用中间表以获取相关ID:

var ids = db.Set<ArticleCompany>().Where(ca => ca.CompanyId == companyEntity.Id).Select(ca => ca.ArticleId);

相应的SQL日志(仅从数据库中提取ID):

exec sp_executesql N'SELECT 
[Extent1].[ArticleId] AS [ArticleId]
FROM [dbo].[ArticlesCompanies] AS [Extent1]
WHERE [Extent1].[CompanyId] = @p__linq__0',N'@p__linq__0 int',@p__linq__0=1