在 Entity Framework 核心中保存多对多关系

Question

例如，我有 3 个类，我将其用于多对多关系：

public class Library
{
    [Key]
    public string LibraryId { get; set; }
    public List<Library2Book> Library2Books { get; set; }
}

public class Book
{
   [Key]
   public string BookId { get; set; }
   public List<Library2Book> Library2Books { get; set; }
}

public class Library2Book
{
    public string BookId { get; set; }
    public Book Book { get; set; }

    public string LibraryId { get; set; }
    public Library Library { get; set; }
}

它们在 ApplicationDbContext 中配置：

protected override void OnModelCreating(ModelBuilder builder)
{
    base.OnModelCreating(builder);
    builder.Entity<CodeableConcept2Coding>().HasKey(k => new { k.LibraryId, k.BookId });
    builder.Entity<Library2Book>()
        .HasOne(x => x.Library)
        .WithMany(x => x.Library2Book)
        .HasForeignKey(x => x.LibraryId);
    builder.Entity<Library2Book>()
        .HasOne(x => x.Book)
        .WithMany(x => x.Library2Book)
        .HasForeignKey(x => x.BookId);
}

所以，我想向数据库中添加一些 Library2Books:

的列表

var library2Books = new List<Library2Books>(/*some sort of initialization*/);

我应该先添加什么实体？ Books 或者 Library？我该如何节省？

Answer 1

对于 EF Core 多对多关系，这是一个简单且非常基础的问题；我不知道为什么没有人在 EF Core 中为 n..m 编写一个完整的示例。

我已经修改了你的代码（主键为int），我不喜欢主键中的字符串。只需 copy/paste 代码，每个代码都可以正常工作。

What entity should I add first? Books or maybe Library? How can I do this saving?

顺序并不重要，这里重要的是数据链接。数据必须正确链接，请参阅我的代码行之间的注释。

备注：

没有实体 class 来表示联接 table 的多对多关系尚不支持！您必须有一个连接 table.

多对多关系由 2 个独立的一对多关系组成。 = 2x 1:N

class Program
{
   public class Library
   {
     [Key]
     public int LibraryId { get; set; }
     public List<Library2Book> Library2Books { get; set; } = new    List<Library2Book>();
   }

   public class Book
   {
     [Key]
     public int BookId { get; set; }
     public List<Library2Book> Library2Books { get; set; } = new List<Library2Book>();
   }

   public class Library2Book
   {
     [Key]
     public int BookId { get; set; }
     public Book Book { get; set; }

     [Key]
     public int LibraryId { get; set; }
     public Library Library { get; set; }
   }

   public class MyDbContext : DbContext
   {
     public DbSet<Book> Books { get; set; }

     public DbSet<Library> Libraries { get; set; }

     protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
     {
       optionsBuilder.UseSqlServer(@"Server=.\;Database=EFTutorial;integrated security=True;");
       base.OnConfiguring(optionsBuilder);
    }

     protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
       modelBuilder.Entity<Library2Book>().HasKey(k => new { k.LibraryId, k.BookId });

       modelBuilder.Entity<Library2Book>()
           .HasOne(x => x.Book)
           .WithMany(x => x.Library2Books)
           .HasForeignKey(x => x.BookId);

       modelBuilder.Entity<Library2Book>()
          .HasOne(x => x.Library)
          .WithMany(x => x.Library2Books)
          .HasForeignKey(x => x.LibraryId);

       base.OnModelCreating(modelBuilder);
     }
   }

   static void Main(string[] args)
   {
     using (var myDb = new MyDbContext())
    {
      // Create Db
       myDb.Database.EnsureCreated();

       // I will add two books to one library
       var book1 = new Book();
       var book2 = new Book();

       // I create the library 
       var lib = new Library();

       // I create two Library2Book which I need them 
       // To map between the books and the library
       var b2lib1 = new Library2Book();
       var b2lib2 = new Library2Book();

       // Mapping the first book to the library.
       // Changed b2lib2.Library to b2lib1.Library
       b2lib1.Book = book1;
       b2lib1.Library = lib;

       // I map the second book to the library.
       b2lib2.Book = book2;
       b2lib2.Library = lib;

       // Linking the books (Library2Book table) to the library
       lib.Library2Books.Add(b2lib1);
       lib.Library2Books.Add(b2lib2);

       // Adding the data to the DbContext.
       myDb.Libraries.Add(lib);

       myDb.Books.Add(book1);
       myDb.Books.Add(book2);

       // Save the changes and everything should be working!
       myDb.SaveChanges();
     }
   }
}

结果

Tables:   Books    |   Libraries      |    Library2Book  |
           1       |      1           |      1   |   1   |
           2       |      -           |      1   |   2   |

问题作者编辑

当您尝试插入大量实体时（我尝试了大约 300 个），您将遇到 已添加相同的键 错误。您应该将插入的集合分成小部分，例如100 个实体应该足够了。

public async Task SaveEntities(IEnumerable<Library2Book> library2Books)
        {
                int i = 0;
                foreach (var library2Book in library2Books)
                {
                    _dbContext.Set<Library>().Add(codConc2Coding.Library);
                    _dbContext.Set<Book>().Add(codConc2Coding.Book);
                    _dbContext.Set<Library2Book>().Add(library2Book);
                    i++;
                    if (i == 99)
                    {
                        await _dbContext.SaveChangesAsync();
                        i = 0;
                    }
                }
                await _dbContext.SaveChangesAsync();
}

在 Entity Framework 核心中保存多对多关系

Saving many-to-many relationship in Entity Framework Core

c#

many-to-many

entity-framework-core