在实体框架核心中保存多对多关系

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; }
}

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);
}

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

这是EF Core多对多关系的一个简单而又非常基本的问题;我不知道为什么没有人在EF Core中为n..m写一个完整的例子。

我修改了你的代码(主键为int)，我不喜欢主键中的字符串。只要复制/粘贴代码，每个都应该正常工作。

我应该先添加哪个实体?书还是图书馆?我怎样才能进行储蓄呢?

顺序不重要，这里重要的是数据链接。数据必须正确链接，参见我的代码行之间的注释。

指出:

• 不支持没有实体类来表示连接表的多对多关系!你必须有一个连接表

• 多对多关系由两个独立的一对多关系组成。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

当你试图插入很多实体(我已经尝试了大约300)，你会有相同的键已经添加错误。你应该把你的插入集合分成小的部分，例如100个实体就足够了。

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();
}