Question

我有一个Books类，其中ApplicationUser为作者。在我的ApplicationUser类中，有一个用户已编写的“书籍”列表（一对多）。

Books类还具有一个ApplicationUsers列表，用于记录喜欢的数量。但是，单个ApplicationUser只能喜欢一本书。当用户喜欢另一本书时，类似的东西会从以前的所有书籍中删除。

public class ApplicationUser : IdentityUser
{
    public string Name { get; set; }

    public virtual ICollection<Book> Books { get; set; }
    public virtual ICollection<Book> Likes { get; set; } //NEW
}

public class Book
{
    public string Id { get; set; }

    public virtual ApplicationUser ApplicationUser { get; set; } //Author

    .......

    public virtual ICollection<ApplicationUser> Likes { get; set; }
}

protected override void OnModelCreating(DbModelBuilder modelBuilder)
    {
        base.OnModelCreating(modelBuilder);

         modelBuilder.Entity<Book>()
                .HasRequired<ApplicationUser>(s => s.ApplicationUser) 
                .WithMany(s => s.Books)
                .WillCascadeOnDelete(true); 

         modelBuilder.Entity<Book>() //NEW
                .HasMany<ApplicationUser>(s => s.Likes)
                .WithMany(s => s.Likes);
  }

关系一定有问题。单个用户只能喜欢一本书。但是用户应该可以喜欢很多书。在检查用户在该书的“喜欢”中是否存在用户之后，book.Likes.Add(user);会简单地添加一个喜欢。

但是Book与创建Book的ApplicationUser之间存在一对多关系。但是，不需要ApplicationUser和Likes之间的关系。

[ResponseType(typeof(void))]
    public IHttpActionResult Like(string bookid, bool action)
    {
        Book book = db.Books.Find(bookid);
        var user = UserManager.FindById(User.Identity.GetUserId());
        if (book == null || user == null)
            return BadRequest();
        if (action)
        {
            if (!book.Likes.Contains(user))
                book.Likes.Add(user);
        }
        else
        {
            if (book.Likes.Contains(user))
                book.Likes.Remove(user);
        }
        //db.Entry(book).State = EntityState.Modified;

        try {
            db.SaveChanges();
        }
        catch (Exception e) {
            throw;
        }
        return StatusCode(HttpStatusCode.NoContent);
    }

更新：因此，按照@Alex Kudryashev的建议，我将Likes集合添加到ApplicationUser类。但是我现在在迁移和更新数据库时遇到此错误。

在表'BookApplicationUsers'上引入FOREIGN KEY约束'FK_dbo.BookApplicationUsers_dbo.AspNetUsers_ApplicationUser_Id'可能会导致循环或多个级联路径。指定ON DELETE NO ACTION或ON UPDATE NO ACTION，或修改其他FOREIGN KEY约束。无法创建约束或索引。查看以前的错误。

Answer 1

导致问题的原因在于您对ApplicationUsers和Books之间的关系不了解。

每个ApplicationUser都创作了零个或多个Books，每个Book被零个或多个ApplicationUsers喜欢。因此ApplicationUsers和Books之间的关系是多对多的。

首先在EF代码中编写如下：

class ApplicationUser
{
    public int Id {get; set;}

    // every ApplicationUser has Authored zero or more books (many-to-many)
    public virtual ICollection<Book> AuthoredBooks {get; set;}

    ... // other properties
}
class Book
{
    public int Id {get; set;}

    // every Book has been Liked by zero or more ApplicationUsers (many-to-many)
    public virtual ICollection<ApplicationUsers> Likers {get; set;}

    ... // other properties
}

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

请参见Configure many-to-many relationship

在MyDbContext.OnModelCreating中：

 modelBuilder.Entity<ApplicationUser>()
     // every applicationUser has zero or more AuthoredBooks
     .HasMany(applicationUser => applicationUser.AuthoredBooks)

     // where every Book has zero or more Likers:
     .WithMany(book => book.Likers)

     // we put the many-to-many in the following junction table:
     .Map(junctionTable=>
     {
         junctionTable.MapLeftKey("ApplicationUserId");
         junctionTable.MapRightKey("BookId");
         junctionTable.ToTable("ApplicationUsersBooks");
      });

尽管创建了联结表，但与多对多表一样，您不会在dbContext中提及它。每当您使用ICollection时，实体框架都会知道GroupJoin需要连接表。

获取所有撰写过真正旧书的年轻ApplicationUsers：

var result = myDbContext.ApplicationUsers
    .Where(user => user.BirtDate > new DateTime(2010, 1, 1)
    .Select(user => new
    {
         Id = user.Id,
         Name = user.Name,
         AuthoredOldBooks = user.Books
             .Where(book => book.PublicationDate < new DateTime(1900, 1, 1)
             .ToList(),
    }

或者获取一个或多个女性喜欢的所有软件书籍：

var results = myDbContext.Books
    .Where(book => book.Genre ==  BookGenre.Software
       && book.Likers.Where(liker => liker.Gender == Gender.Female).Any());

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1 个答案: