EF 检查集合初始化状态

Question

让我们假设以下类：

public class Author
{
    public virtual string Name { get; set; }
    public virtual List<Book> Books { get; set; }
} 

public class Book
{
    public virtual string Name { get; set; }
    public virtual Author Author { get; set; }
}

public class Controller
{
    public void DeleteBook(Book book)
    {
        var author = book.Author; //first check if it is loaded, not to invoke lazy loading?
        author.Books.Remove(book) //check if the books collection is loaded?
        book.Author = null;

        Context.Set<Book>().Remove(book);        
    }
}

我的问题是 - EF 有没有办法检查两个“已加载”状态？ 我想确保书籍作者和书籍收藏不会被加载只是为了解除关联。

我想写这样的东西：

public class Controller
{
    public void DeleteBook(Book book)
    {
        if (EF.IsLoaded(book.Author)) //has it been (lazy) loaded / initialized?
        {
             if (EF.IsLoaded(book.Author.Books) //has it been (lazy) loaded / initialized?
             {
                 book.Author.Books.Remove(book);
             }
             book.Author = null;
        }
        Context.Set<Book>().Remove(book);        
    }
}

这可能吗？

Answer 1

我知道您可以使用RelationshipManager 检查集合是否已加载，该RelationshipManager 将适用于任何包含关系的实体，并实现IEntityWithRelationships。

 var result = ((IEntityWithRelationships)(author)).RelationshipManager
       .GetRelatedCollection<Books>("FK_Authors_Books", "Books");

   if (result.IsLoaded == false)
   {
       // do something here
   }

注意：您必须用正确的外键名称代替“FK_Authors_Books”。

我相信您也可以使用 GetRelatedReference 对单个对象执行相同的操作。

 var result = ((IEntityWithRelationships)(book)).RelationshipManager
       .GetRelatedReference<Author>("FK_Books_Author", "Author");

   if (result.IsLoaded == false)
   {
       // do something here
   }

Answer 2

在您的 DBContext 中，您可以检查是否加载了访问属性 Local 的对象，例如：DBContext.Book.Local

我认为你可以做这样的事情

public IsLoaded(Book book)
{
    DBContext.Book.Local.Count(b => b.ID == book.ID) > 0;
}

Answer 3

请记住，我不是 EF 专家，但也许它会给你一个想法。 DbContext.Entry 方法使您可以访问 DBContext 拥有的有关实体的所有信息。据我了解，您想知道传递的对象（作为参数）是否已经在内存中？

另外请记住，下面的方法是使用explicit loading。

public class Controller
{
    public void DeleteBook(Book book)
    {
        // assuming that you're working on context directly and need to dispose it
        using (var context = new YourContext())
        {
            var entry = context.Entry(book);
            // Will tell you if Author navigation property is loaded
            bool isLoaded = entry.Reference(x => x.Author).IsLoaded();

            if (isLoaded != false)
            {
                // do when Author navigation property is loaded
            }
            else
            {
                // do when Author navigation property is not loaded
            }
        }
     }
}

对于 Collection 类型的导航属性，您只需使用 .Collection 而不是 .Reference。

http://msdn.microsoft.com/en-us/library/system.data.objects.dataclasses.relatedend.isloaded(v=vs.110).aspx