Question

我的数据库中设置了多对多关系，如下所示：

User
-------
Id (PK, Identity)
First
Last
...various other fields

Skill
-------
Id (PK, Identity)
Description

UserSkill
-----------
UserId (PK, FK on User.Id)
SkillId (PK, FK On Skill.Id)

当我在DbContext上运行此LINQ查询时：

from u in Users 
from s in u.Skills 
where s.Id == 5 
select new 
{
    u.Id,
    s.Description
})

生成的SQL包含我想要的所有内连接：

SELECT 
[Extent1].[UserId] AS [UserId], 
[Extent2].[Description] AS [Description]
FROM  [dbo].[UserSkill] AS [Extent1]
INNER JOIN [dbo].[Skill] AS [Extent2] ON [Extent1].[SkillId] = [Extent2].[Id]
WHERE 5 = [Extent2].[Id]

但是，当我添加一个简单的额外where子句时：

from u in Users 
from s in u.Skills 
where s.Id == 5 
    && u.Last == "test"
select new 
{
    u.Id,
    s.Description
})

现在生成的SQL使用子查询：

[Extent1].[Id] AS [Id], 
[Filter1].[Description] AS [Description]
FROM  [dbo].[User] AS [Extent1]
INNER JOIN  (SELECT [Extent2].[UserId] AS [UserId], [Extent3].[Description] AS [Description]
    FROM  [dbo].[UserSkill] AS [Extent2]
    INNER JOIN [dbo].[Skill] AS [Extent3] ON [Extent3].[Id] = [Extent2].[SkillId]
    WHERE 5 = [Extent3].[Id] ) AS [Filter1] ON [Extent1].[Id] = [Filter1].[UserId]
WHERE 'test' = [Extent1].[Last]

也许我错过了一些东西，但我认为EF只会将另一个连接添加回User查询的User表，并且能够在User.Last上执行某个操作而不是执行子查询。有没有办法强迫这种行为？难道我做错了什么？

感谢。

更新

Cosmin，我希望查询能够像这样出现：

SELECT u.Id, s.Description
FROM [User] u INNER JOIN
        [UserSkill] us ON u.Id = us.UserId INNER JOIN
        [Skill] s ON us.SkillId = s.Id
WHERE s.Id = 2 AND u.Last = 'test'

Answer 1

看起来这是EF目前不做的优化。就个人而言，除非性能成为问题，否则我会坚持使用它生成的子查询。

但是如果您愿意丢失用户和技能的直接导航属性，则可以对中间表进行建模以获取您要查找的查询。

public class User
{
    public int Id { get; set; }
    public string First { get; set; }
    public string Last { get; set; }

    public virtual ICollection<UserSkill> UserSkills { get; set; }
}

public class UserSkill
{
    public int Id { get; set; }
    [Required]
    public User User { get; set; }
    [Required]
    public Skill Skill { get; set; }

}

public class Skill
{
    public int Id { get; set; }
    public string Description { get; set; }

    public virtual ICollection<UserSkill> UserSkills { get; set; }
}

然后，以下查询将生成连接而不是子查询

 from x in db.UserSkills 
 where x.Skill.Id == 5 && x.User.Last == "test"
 select new {x.User.Id, x.Skill.Description};

Answer 2

@ryanulit，您的问题是有效的，这对所有Linq to Entities都是一个问题。请查看已发布的MS论坛网址

MS Forum's URL

使用多对多关系实体框架强制内部联接

2 个答案: