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);
}

因此,我想将Library2Books的一些列表添加到数据库中:
var library2Books = new List<Library2Books>(/*some sort of initialization*/);

我应该先添加哪个实体? Books还是Library?我该如何保存?

最佳答案

对于EF Core多对多关系来说,这是一个简单且非常基础的问题;我不知道为什么没人在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,即可正常工作。



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

笔记:

  • 不支持没有实体类来表示联接表的多对多关系!您必须有一个联接表。
  • 多对多关系由2个单独的一对多关系组成。 = 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   |
    

    由问题的作者编辑

    当您尝试插入许多实体(我尝试过大约300个)时,您将拥有已被添加相同键的错误。您应该将插入的收藏分成小部分,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();
    }
    

    关于c# - 在Entity Framework Core中保存多对多关系,我们在Stack Overflow上找到一个类似的问题:https://stackoverflow.com/questions/38893873/

    10-13 09:13