Question

public class Book
    {
        public string Name { get; set; }        
        public string Description { get; set; }
        public double Price { get; set; }
        public int Stock { get; set; }
        public DateTime DateAdded { get; set; }        
        public int AuthorId { get; set; }
        public  Author Author { get; set; }
        public int CategoryId { get; set; }
        public  Category Category { get; set; }
     }

通过急切加载查询数据库时，如果其包含的嵌套类之一为null，则返回model null。我有一个图书实体，该实体具有两个从属类-作者和类别。我想标识为null的嵌套实体，并将其从查询中排除。如果它们都为null，则将它们都排除，并仅返回给定id的Book。我写出了以下lambda查询，但它看起来有点粗糙，对数据库的攻击次数太多，需要简化。有没有更好的方法可以使此逻辑看起来更加优雅和简化？

public ActionResult ProductView (int? id)
{
    if (id != null)
    {
        var book = db.Books.Where(b => b.Id == id).Include(b => b.Author).Include(b=> b.Category.Books).SingleOrDefault();
        if(book==null)
        {
            var bookWithCtg = db.Books.Where(b => b.Id == id).Include(b => b.Category).SingleOrDefault();
            var bookWithAuth = db.Books.Where(b => b.Id == id).Include(b => b.Author).SingleOrDefault();

            if (bookWithCtg == null&& bookWithAuth ==null)
                book = db.Books.Where(b => b.Id == id).SingleOrDefault();

            else if(bookWithAuth==null)
                book = db.Books.Where(b => b.Id == id).Include(b => b.Category.Books).SingleOrDefault();

            else if (bookWithCtg == null)
                book = db.Books.Where(b => b.Id == id).Include(b => b.Author).SingleOrDefault();
        }

        book.DisplayNumber++;
        db.SaveChanges();
        return View(book);
    }

    else
       return HttpNotFound();
}

Answer 1

由于您的导航属性配置，在连接书籍，作者和类别时，EntityFramework使用内部连接。如果一本书的作者和类别可以为空，则必须将AuthorId和CategoryId定义为 nullable int ，并按如下所示更改您的实体类型配置：

HasOptional(b => b.Author)
 .WithMany(a => a.Books)
 .HasForeignKey(b => b.AuthorId)

HasOptional(b => b.Category)
 .WithMany(a => a.Books)
 .HasForeignKey(b => b.CategoryId)

完成此配置后，您只需添加“作者”和“类别”即可。

var book = db.Books
 .Where(b => b.Id == id)
 .Include(b => b.Author)
 .Include(b=> b.Category)
 .SingleOrDefault();

现在，此查询将返回图书数据，无论作者和类别如何。

确定模型中哪个嵌套属性为null？

1 个答案: