實體框架6：Skip（）＆Take（）不生成SQL，而是在加載到內存後過濾結果集。或者我做錯了什麼？

Question

我有以下代碼應該得到一些book，並檢索該書（Book實體）的前兩個tag（Tag實體）。 因此Tags是Book實體的navigation property。

using (var context = new FakeEndavaBookLibraryEntities()) 
{ 
     Book firstBook = context.Set<Book>().Take(1).First(); 
     var firstTwoTags = firstBook.Tags.OrderBy(tag => tag.Id).Skip(0).Take(2).ToList(); 
} 

我預計獲得有由EF生成以下SQL查詢。

SELECT TOP(2) 
     [Extent2].[Id]  AS [Id], 
     [Extent2].[Version] AS [Version], 
     [Extent2].[Name] AS [Name] 
FROM [Literature].[BookTagRelation] AS [Extent1] 
     INNER JOIN [Literature].[Tag] AS [Extent2] 
     ON [Extent1].[TagId] = [Extent2].[Id] 
WHERE [Extent1].[BookId] = 1 /* @EntityKeyValue1 - [BookId] */ 

取而代之的是，EF探查表明我的EF是產生無限的結果集（如SELECT * FROM ...）

SELECT [Extent2].[Id]  AS [Id], 
     [Extent2].[Version] AS [Version], 
     [Extent2].[Name] AS [Name] 
FROM [Literature].[BookTagRelation] AS [Extent1] 
     INNER JOIN [Literature].[Tag] AS [Extent2] 
     ON [Extent1].[TagId] = [Extent2].[Id] 
WHERE [Extent1].[BookId] = 1 /* @EntityKeyValue1 - [BookId] */ 

Here is a scheme fragment if you need it

我也試圖追加.AsQueryable()至firstBook.Tags財產和/或刪除.Skip(0)方法，如下所示，但這也沒有幫助。

 var firstTwoTags = firstBook.Tags.AsQueryable().OrderBy(tag => tag.Id).Skip(0).Take(2).ToList(); 

同樣意外的行爲：

SELECT [Extent2].[Id]  AS [Id], 
     [Extent2].[Version] AS [Version], 
     [Extent2].[Name] AS [Name] 
FROM [Literature].[BookTagRelation] AS [Extent1] 
     INNER JOIN [Literature].[Tag] AS [Extent2] 
     ON [Extent1].[TagId] = [Extent2].[Id] 
WHERE [Extent1].[BookId] = 1 /* @EntityKeyValue1 - [BookId] */ 

---

你曾經與實體框架6工作時遇到同樣的問題？

是否有任何解決方法來克服這個問題，或者我設計了一個錯誤的方式查詢......？

感謝您的任何提示！

Answer 1

正如@hvd指出，我與IQueryable<Tag>工作，而firstBook.Tags導航屬性僅僅是一個懶惰的加載IEnumerable<Tag>。 所以這裏是我的問題的解決方案，基於@hvd的答案。

Tag firstTag = context.Set<Tag>() // or even context.Tags 
    .Where(tag => tag.Books.Any(book => book.Id == firstBook.Id)) 
    .OrderBy(tag => tag.Id) 
    .Skip(0).Take(1) 
    .SingleOrDefault(); 

所以@hvd的溶液的微小的變化是：與

東西EF理解

1）.Where(tag => tag.Books.Any(book => book.Id == firstBook.Id))替換的

.Where(tag => tag.Books.Contains(firstBook))。

或

2）.Where(tag => tag.Books.Select(book => book.Id).Contains(firstBook.Id))

的代碼任何序列（1）或（2）將生成以下SQL查詢，這絕對是不再一個無限結果集。

SELECT [Project2].[Id]  AS [Id], 
     [Project2].[Version] AS [Version], 
     [Project2].[Name] AS [Name] 
FROM (SELECT [Extent1].[Id]  AS [Id], 
       [Extent1].[Version] AS [Version], 
       [Extent1].[Name] AS [Name] 
     FROM [Literature].[Tag] AS [Extent1] 
     WHERE EXISTS (SELECT 1 AS [C1] 
         FROM [Literature].[BookTagRelation] AS [Extent2] 
         WHERE ([Extent1].[Id] = [Extent2].[TagId]) 
           AND ([Extent2].[BookId] = 1 /* @p__linq__0 */))) AS [Project2] 
ORDER BY [Project2].[Id] ASC 
OFFSET 0 ROWS 
FETCH NEXT 1 ROWS ONLY 

Answer 2

firstBook.Tags是一個懶惰加載的IEnumerable<Tag>。在第一次訪問時，所有標籤都被加載，隨後將其變爲IQueryable<Tag>的嘗試不起作用，因爲您並非從可以明智地查詢的東西開始。

相反，從已知的好IQueryable<Tag>開始。東西沿線

Tag firstTag = context.Set<Tag>() 
    .Where(tag => tag.Books.Contains(firstBook)) 
    .OrderBy(tag => tag.Id).Skip(0).Take(1).SingleOrDefault(); 

應該工作。您可能需要稍微調整以將過濾條件轉換爲EF理解的內容。