在 Entity Framework Core 中保存多对多关系

Question

例如，我有 3 个类，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，我想在数据库中添加一些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？我怎样才能做到这一点？

Answer 1

这是一个关于 EF Core 多对多关系的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 编写完整的示例。

我修改了你的代码（主键为int），我不喜欢主键中的字符串。只需复制/粘贴代码，每个都可以正常工作。

我应该先添加什么实体？书籍或图书馆？我怎样才能做到这一点？

顺序不重要，重要的是数据链接。数据必须正确链接，查看我的代码行之间的 cmets。

注意事项：

• 尚不支持没有实体类来表示连接表的多对多关系！您必须有一个连接表。

• 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();
}