Question

假设我在Entity Framework，Library和Book中有两个表。这些是多对多的连接，在另一个名为Library_Book的表中，但它被隐藏，因为它是连接表。

如何通过查询连接表（实体框架中没有自己的类）来检查库是否有某本书，而不从连接表中返回库中所有书籍的列表，而是使用LINQ检查只有具有bookId和LibraryId的记录。

非常感谢提前。

Answer 1

要确定已知图书馆是否有已知图书，您可以执行以下操作：

bool hasIt = (from l in Libraries from b in l.Books where b.Id == 5 where l.Id == 3 
select l).Any();

此Linq查询不会触及Library或Books表。它看起来像这样：

    SELECT 
CASE WHEN ( EXISTS (SELECT 
    1 AS [C1]
    FROM [dbo].[LibraryBookIntersection] AS [Extent1]
    WHERE (5 = [Extent1].[BookId]) AND (3 = [Extent1].[LibraryId])
)) THEN cast(1 as bit) WHEN ( NOT EXISTS (SELECT 
    1 AS [C1]
    FROM [dbo].[LibraryBookIntersection] AS [Extent2]
    WHERE (5 = [Extent2].[BookId]) AND (3 = [Extent2].[LibraryId])
)) THEN cast(0 as bit) END AS [C1]
FROM  ( SELECT 1 AS X ) AS [SingleRowTable1]

在实体框架中询问多对多的表

1 个答案: