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，我想向数据库添加一些Library2Books列表：

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

我应该先添加什么实体？ Books或Library ？ 我怎样才能做到这一点？

Answer 1

这是 EF Core 多对多关系的一个简单且非常基本的问题； 我不知道为什么没有人在 EF Core 中为 n..m 编写完整的示例。

我已经修改了您的代码（主键为int ），我不喜欢主键中的字符串。 只需复制/粘贴代码，每个代码都可以正常工作。

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。

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！ 你必须有一个连接表。

• 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   |

问题作者的编辑

当您尝试插入大量实体时（我已经尝试了大约 300 个），您将遇到相同的键已添加错误。 您应该将您的插入集合分成小部分，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();
}