Question

我意识到我并不完全理解LINQ-to-Entities中的Include方法。

例如，请参阅下面的两个代码段。我希望它们能够产生相同的输出（尽管第一个版本可能更有效，因为它避免了JOIN）。

// 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 />");
  }
}

但每个代码段的输出都不同：

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

第一个代码段正确输出{Edition Id} - {Edition Title}，而第二个代码段意外地打印{Book Id} - {Edition Title}，并且仅显示每本书的第一版。

发生了什么事？有没有办法使用Include来实现所需的输出？

编辑1 ：MySql数据看起来像（已更正）：

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 } }

请注意，Edition或Id = 8没有Id = 9。

上面的代码是我的完整代码，在Page_Load中是一个空的测试页。

编辑2 ：我测试过以下内容并没有什么区别：

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

编辑3 ：如果启用延迟加载，则以下工作（在代码段2中）：

var author = db.Authors.First();

（我猜这基本上和Snippet 1一样。）

然而，无论延迟加载如何，这仍会返回奇怪的输出：

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

编辑4 ：我很抱歉，但我误解了上面的表格结构。（我现在有一天。）现在纠正了，以显示多对多的关系。请参阅编辑1。

也是

((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

CASE语句很有意思，因为我的MySql字段都不可为空。

Answer 1

实体框架的提供程序在LINQ语句中编译First()表达式时可能存在错误。涉及Include时偶尔会出现奇怪现象：http://wildermuth.com/2008/12/28/Caution_when_Eager_Loading_in_the_Entity_Framework

尝试将第二个代码段重写为：

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 />");
        }
    }
}

如果这样可以修复输出，那么肯定是First()方法。

编辑：你对第三次编辑做正确与你的代码片段1相同是正确的。我无法理解这种包含语句是如何严重绊倒的。我唯一能鼓励的是查看它生成的SQL查询：

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

编辑2：考虑到生成疯狂的sql输出，很可能问题出现在你的EF MySQL提供程序中。

包含LINQ到实体的奇怪行为

1 个答案: