與LINQ到實體的奇怪行爲包括

Question

我意識到我不完全理解LINQ-to-Entities中的Include方法。

例如，下面的兩個代碼片段。我期望他們產生相同的輸出（儘管第一個版本可能更高效，因爲它避免了JOIN s）。

// Snippet 1 
using (var db = new Db()) { 
    var author = db.Authors.First(); 
    db.LoadProperty<Author>(author, o => o.Books); 
    foreach (var book in author.Books) { 
    db.LoadProperty<Book>(book, o => o.Editions); 
    foreach (var edition in book.Editions) 
     Response.Write(edition.Id + " - " + edition.Title + "<br />"); 
    } 
} 

Response.Write("<br />"); 

// Snippet 2 
using (var db = new Db()) { 
    var author = db.Authors.Include("Books.Editions").First(); 
    foreach (var book in author.Books) { 
    foreach (var edition in book.Editions) 
     Response.Write(edition.Id + " - " + edition.Title + "<br />"); 
    } 
} 

但輸出爲每個代碼段不同的是：

1 - Some Book First Edition 
2 - Another Book First Edition 
3 - Another Book Second Edition 
4 - Another Book Third Edition 

8 - Some Book First Edition 
9 - Another Book First Edition 

第一片段正確地輸出{Edition Id} - {Edition Title}，而第二個意外打印{Book Id} - {Edition Title}和只給出了每本書的第一版本。

發生了什麼事？有沒有一種方法可以使用Include來實現所需的輸出？

EDIT 1：MySQL數據看起來像（校正）：

Authors   = { { Id = 1, Name = "Some Author" } } 

Books   = { { Id = 8, AuthorId = 1 }, 
        { Id = 9, AuthorId = 1 } } 

Editions  = { { Id = 1, Title = "Some Book First Edition" }, 
        { Id = 2, Title = "Another Book First Edition" }, 
        { Id = 3, Title = "Another Book Second Edition" }, 
        { Id = 4, Title = "Another Book Third Edition" } } 

EditionsInBooks = { { BookId = 8, EditionId = 1 }, 
        { BookId = 9, EditionId = 2 }, 
        { BookId = 9, EditionId = 3 }, 
        { BookId = 9, EditionId = 4 } } 

注意，沒有用Edition或Id = 8​​。

而上面的代碼是我的完整代碼，在Page_Load中爲空的測試頁。

編輯2：我已經測試了以下，他們不有所作爲：

1. var author = db.Authors.Include("Books.Editions").AsEnumerable().First();
2. var author = db.Authors.Include("Books.Editions").Single(o => o.Id == 1);
3. var author = db.Authors.Include("Books").Include("Books.Editions").First();

編輯3：如果我啓用延遲加載，以下工作（在代碼段2中）：

var author = db.Authors.First(); 

（這實質上執行相同片段1，我想。）

然而，這仍返回奇怪輸出而不管延遲加載：

var author = db.Authors.Include("Books.Editions").First(); 

EDIT 4 ：我很抱歉，但我錯誤地表述了上面的表格結構。 （我有一個這樣的日子。）現在已經糾正了，以顯示多對多的關係。請參閱編輯1.

而且輸出爲

((ObjectQuery)db.Authors.Include("Books.Editions").AsEnumerable()) 
    .ToTraceString() 

是

SELECT 
    `Project1`.`Id`, 
    `Project1`.`Name`, 
    `Project1`.`C2` AS `C1`, 
    `Project1`.`id1`, 
    `Project1`.`AuthorId`, 
    `Project1`.`C1` AS `C2`, 
    `Project1`.`id2`, 
    `Project1`.`Title` 
FROM (SELECT 
    `Extent1`.`Id`, 
    `Extent1`.`Name`, 
    `Join2`.`Id` AS `id1`, 
    `Join2`.`AuthorId`, 
    `Join2`.`Id` AS `id2`, 
    `Join2`.`Title`, 
    CASE WHEN (`Join2`.`Id` IS NULL) THEN (NULL) 
     WHEN (`Join2`.`BookId` IS NULL) THEN (NULL) 
     ELSE (1) END AS `C1`, 
    CASE WHEN (`Join2`.`Id` IS NULL) THEN (NULL) 
     ELSE (1) END AS `C2` 
    FROM `authors` AS `Extent1` 
    LEFT OUTER JOIN (SELECT 
     `Extent2`.`Id`, 
     `Extent2`.`AuthorId`, 
     `Join1`.`BookId`, 
     `Join1`.`EditionId`, 
     `Join1`.`Id` AS `Id1`, 
     `Join1`.`Title` 
     FROM `books` AS `Extent2` 
     LEFT OUTER JOIN (SELECT 
     `Extent3`.`BookId`, 
     `Extent3`.`EditionId`, 
     `Extent4`.`Id`, 
     `Extent4`.`Title` 
     FROM `editionsinbooks` AS `Extent3` 
     INNER JOIN `editions` AS `Extent4` 
      ON `Extent4`.`Id` = `Extent3`.`EditionId`) AS `Join1` 
     ON `Extent2`.`Id` = `Join1`.`BookId`) AS `Join2` 
    ON `Extent1`.`Id` = `Join2`.`AuthorId`) AS `Project1` 
    ORDER BY 
    `Project1`.`Id` ASC, 
    `Project1`.`C2` ASC, 
    `Project1`.`id1` ASC, 
    `Project1`.`C1` ASC 

的CASE語句是有趣的，因爲沒有我的MySQL字段是空。

Answer 1

實體框架的提供者在LINQ語句中編譯First()表達式時可能存在一個錯誤。還有當Include參與偶然的怪事：http://wildermuth.com/2008/12/28/Caution_when_Eager_Loading_in_the_Entity_Framework

請嘗試重寫第二個片段是：

using (var db = new Db()) { 
    var author = db.Authors.Include("Books.Editions").AsEnumerable().First(); 
    foreach (var book in author.Books) 
    { 
     foreach (var edition in book.Editions) 
     { 
      Response.Write(edition.Id + " - " + edition.Title + "<br />"); 
     } 
    } 
} 

如果修復您的輸出，那麼它絕對是First()方法。

編輯：你是正確的關於你的第三個編輯做與片段1相同的事情。我無法理解包含語句是如何嚴重地讓事情變得糟糕。我鼓勵的唯一的事情是看看SQL查詢它的產生：

var sql = ((System.Data.Objects.ObjectQuery)db.Authors.Include("Books.Editions").AsEnumerable().First()).ToTraceString(); 

編輯2：這很可能是可能的，問題是出在你的MySQL提供商EF，給瘋狂的SQL輸出產生。