实体框架 4.1 Code-First 方法在域服务上实现多对多关系

Question

我在使用最新的实体框架和代码优先创建数据库模型时遇到了一些问题（有关详细信息，请参阅Entity Framework 4.1 Code First approach to create many-to-many relation）。

同时我发现问题不再是实体框架本身，而是与 WCF RIA DomainServices 一起使用。

为了完整起见 - 这是我的相关代码优先代码：

//
// Models
//

public class Author
{
    public Author()
    {
        this.Books = new Collection<Book>();
    }

    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    [Key]
    public int ID { get; set; }

    [MaxLength(32)]
    [Required]
    public string Name { get; set; }

    [Include]
    [Association("Author_Book", "ID", "ID")]
    public Collection<Book> Books { get; set; }
}

public class Book
{
    public Book()
    {
        // this.Authors = new Collection<Author>();
    }

    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    [Key]
    public int ID { get; set; }

    [MaxLength(32)]
    [Required]
    public string Name { get; set; }

    // I really would like to create this navigation property, but there seems to be no way
    // to tell my DomainService to include it.
    // public Collection<Author> Authors { get; set; }
}


//
// Mappings
//

public class AuthorMapping : EntityTypeConfiguration<Author>
{
    public AuthorMapping()
        : base()
    {
        this.HasMany (g => g.Books)
            .WithMany(/*m => m.Authors*/)
            .Map     (gm => gm.ToTable("Author_Book"));
    }
}


//
// DbContext
//

public class BookAuthorModelContext : DbContext
{
    public BookAuthorModelContext()
        : base(@"data source=localhost\MSSQLSERVER2008R2;database=BookAuthor;integrated security=True;")
    {
    }


    public DbSet<Author> Authors  { get; set; }
    public DbSet<Book>   Books { get; set; }


    protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        modelBuilder.Configurations.Add(new AuthorMapping());
        modelBuilder.Conventions.Remove<PluralizingTableNameConvention>();
    }
}


//
// DomainService
//

[EnableClientAccess()]
public class BookAuthorDomainService : DomainService
{
    static BookAuthorDomainService()
    {
        Database.SetInitializer<BookAuthorModelContext>(new BookAuthorModelInitializer());
    }

    public BookAuthorDomainService()
    {
        this.m_modelContext = new BookAuthorModelContext();
    }


    public IQueryable<Author> GetAuthors()
    {
        return this.m_modelContext.Authors.Include("Books");
    }

    public void InsertAuthor(Author Author)
    {
        this.m_modelContext.Insert(Author);
    }

    public void UpdateAuthor(Author Author)
    {
        this.m_modelContext.Update(Author, this.ChangeSet.GetOriginal(Author));
    }

    public void DeleteAuthor(Author Author)
    {
        this.m_modelContext.Delete(Author);
    }


    public IQueryable<Book> GetBooks()
    {
        return this.m_modelContext.Books;//.Include("Authors");
    }

    public void InsertBook(Book Author)
    {
        this.m_modelContext.Insert(Author);
    }

    public void UpdateBook(Book Author)
    {
        this.m_modelContext.Update(Author, this.ChangeSet.GetOriginal(Author));
    }

    public void DeleteBook(Book Author)
    {
        this.m_modelContext.Delete(Author);
    }


    protected override void Dispose(bool disposing)
    {
        if (disposing)
            this.m_modelContext.Dispose();
        base.Dispose(disposing);
    }

    protected override bool PersistChangeSet()
    {
        this.m_modelContext.SaveChanges();
        return base.PersistChangeSet();
    }


    private BookAuthorModelContext m_modelContext;
}

SQL 表按预期创建。在我的客户端应用程序中，我使用带有 DomainDataSource 的 RadGridView：

<UserControl>
    <UserControl.Resources>
        <webServices:BookAuthorDomainContext x:Name="BookAuthorDomainContext"/>
    </UserControl.Resources>

    <riaControls:DomainDataSource x:Name="AuthorDomainDataSource"
                                  DomainContext="{StaticResource BookAuthorDomainContext}" QueryName="GetAuthorsQuery"                                
                                  d:DesignData="{d:DesignInstance webModels:Author, CreateList=true}">

    <telerik:RadGridView x:Name="AuthorGridView" DataContext="{Binding ElementName=AuthorDomainDataSource}" 
                         ItemsSource="{Binding Data}" IsBusy="{Binding IsBusy}"/>
</UserControl>

现在事情变得有趣了。如果我将两条记录添加到空数据库——一条添加到 Author 表，一条添加到 Book 表——那么两条记录的“ID”字段都是“1”。有趣的是，包含 Books 的 GetAuthorsQuery() 将 Book 添加到 Authors 'Books' 属性中。 创建的 Author_Book (join-) 表中没有条目。所以，我已经启动了我的 SQL-Profiler 来看看这里到底发生了什么。这就是我发现的：

SELECT 
[Project1].[ID] AS [ID], 
[Project1].[Name] AS [Name], 
[Project1].[C1] AS [C1], 
[Project1].[ID1] AS [ID1], 
[Project1].[Name1] AS [Name1]
FROM ( SELECT 
    [Limit1].[ID] AS [ID], 
    [Limit1].[Name] AS [Name], 
    [Join1].[ID] AS [ID1], 
    [Join1].[Name] AS [Name1], 
    CASE WHEN ([Join1].[Author_ID] IS NULL) THEN CAST(NULL AS int) ELSE 1 END AS [C1]
    FROM   (SELECT TOP (20) [c].[ID] AS [ID], [c].[Name] AS [Name]
        FROM [dbo].[Author] AS [c] ) AS [Limit1]
    LEFT OUTER JOIN  (SELECT [Extent2].[Author_ID] AS [Author_ID], [Extent3].[ID] AS [ID], [Extent3].[Name] AS [Name]
        FROM  [dbo].[Author_Book] AS [Extent2]
        INNER JOIN [dbo].[Book] AS [Extent3] ON [Extent3].[ID] = [Extent2].[Book_ID] ) AS [Join1] ON [Limit1].[ID] = [Join1].[Author_ID]
)  AS [Project1]
ORDER BY [Project1].[ID] ASC, [Project1].[C1] ASC

他为什么要这样做？我真的很想使用我的多对多关系，但我也很乐意使用单向关系（至少有些东西会起作用）。

提前感谢您的帮助。

Answer 1

我使用的不是属性，而是地图。无论如何，我希望你会发现它有用。

这就是我在作者和书籍之间编写多对多关系的方式，并且能够从作者那里访问一本书，反之亦然。

以下是一个完整的例子，你可以复制粘贴编译。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using WordAndImages.Entities;
using System.Data.Entity;
using System.Data.Entity.ModelConfiguration;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Data.Entity;
using System.Data.Entity.ModelConfiguration;
using System.ComponentModel.DataAnnotations;

namespace Bookstore
{
    public class Author
    {
        public int Id { get; set; }
        public string Name { get; set; }
        public virtual ICollection<Book> Books { get; set; }
        public Author()
        {
            Books = new List<Book>();
        }
    }

    public class Book
    {
        public int Id { get; set; }
        public string Title { get; set; }
        public virtual ICollection<Author> Authors { get; set; }
        public Book()
        {
            Authors = new List<Author>();
        }
    }

    public class Context : DbContext
    {
        static Context()
        {
            Database.SetInitializer<Context>(null);
        }

        public DbSet<Author> Authors { get; set; }
        public DbSet<Book> Books { get; set; }

        protected override void OnModelCreating(DbModelBuilder modelBuilder)
        {
            modelBuilder.Configurations.Add(new AuthorMap());
            modelBuilder.Configurations.Add(new BookMap());
        }
    }

    public class BookMap : EntityTypeConfiguration<Book>
    {
        public BookMap()
        {
            this.HasMany(t => t.Authors)
            .WithMany(a => a.Books)
            .Map(t => t.ToTable("authorsbooks").MapLeftKey("book_id").MapRightKey("author_id"));
        }
    }

    public class AuthorMap : EntityTypeConfiguration<Author>
    {
        public AuthorMap()
        {
            this.HasMany(a => a.Books)
            .WithMany(b => b.Authors)
            .Map(t => t.ToTable("authorsbooks").MapLeftKey("author_id").MapRightKey("book_id"));
        }
    }

    class Program
    {
        static void Main(string[] args)
        {

            #region Saving

            var context = new Context();
            context.Database.Delete();
            context.Database.CreateIfNotExists();

            var book1 = new Book { Title = "Joy" };
            var book2 = new Book { Title = "Happy" };

            var author1 = new Author { Name = "Lisa" };
            var author2 = new Author { Name = "John" };
            var author3 = new Author { Name = "Luca" };

            book1.Authors.Add(author1);
            book1.Authors.Add(author2);

            book2.Authors.Add(author1);
            book2.Authors.Add(author3);

            context.Books.Add(book1);
            context.Books.Add(book2);
            context.SaveChanges();

            #endregion

            #region Accessing a book from it's author and viceversa

            var context2 = new Context();


            var recovered_book1 = context2.Books.Where(b => b.Title == "Joy").FirstOrDefault();
            Console.WriteLine(string.Format("Book1 has title {0} and has {1} authors", recovered_book1.Title, recovered_book1.Authors.Count));
            foreach (var author in recovered_book1.Authors)
                Console.WriteLine(author.Name);

            var recovered_book2 = context2.Books.Where(b => b.Title == "Joy").FirstOrDefault();
            Console.WriteLine(string.Format("Book2 has title {0} and has {1} authors", recovered_book2.Title, recovered_book2.Authors.Count));
            foreach (var author in recovered_book1.Authors)
                Console.WriteLine(author.Name);


            var recovered_author1 = context2.Authors.Where(a => a.Name == "Lisa").FirstOrDefault();
            Console.WriteLine(string.Format("{0} wrote {1} books", recovered_author1.Name, recovered_author1.Books.Count));
            foreach (var book in recovered_author1.Books)
                Console.WriteLine(book.Title);

            Console.ReadLine();
            #endregion
        }
    }
}

当从数据库中恢复一本书时，它会运行这个查询

SELECT TOP (1) 
[Extent1].[Id] AS [Id], 
[Extent1].[Title] AS [Title]
FROM [dbo].[Books] AS [Extent1]
WHERE N'Joy' = [Extent1].[Title]

当它恢复（通过延迟加载）其作者时，它会运行

exec sp_executesql N'SELECT 
[Extent2].[Id] AS [Id], 
[Extent2].[Name] AS [Name]
FROM  [dbo].[authorsbooks] AS [Extent1]
INNER JOIN [dbo].[Authors] AS [Extent2] ON [Extent1].[author_id] = [Extent2].[Id]
WHERE [Extent1].[book_id] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=1