EF双向一对一中的坑

EF版本 6.0

在项目中双向一对一关系是普遍存在的,如果不仔细检查,并不容易发现这个坑

下面新建两个类(假设这两个类是一对一的关系)对应实体都设置为可延迟加载

EF双向一对一中的坑

EF双向一对一中的坑

映射关系为:

EF双向一对一中的坑

EF双向一对一中的坑

再建一个数据访问类:

EF双向一对一中的坑

运行下面查询语句:

TestContext db = new TestContext();

 var list = db.QuerySchool.ToList();

正常情况下,生成的查询语句应该只查询SchoolInfo表,但是生成sql语句却是这样:

SELECT

    [Extent1].[Sid] AS [Sid],

    [Extent1].[SchoolName] AS [SchoolName],

    [Extent2].[Id] AS [Id]

    FROM  [dbo].[SchoolInfo] AS [Extent1]

    LEFT OUTER JOIN [dbo].[ClassInfo] AS [Extent2] ON [Extent1].[Sid] = [Extent2].[SchoolId]

进行了一个表联查,明明没有查询ClassInfo表这里却进行了关联(问题1)

下面修改下查询语句

var list = db.QuerySchool.Select(x => new { SId = x.Sid, SchoolName = x.SchoolName }).ToList();

只查询出ID和SchoolName,生成sql语句如下:

SELECT

    [Extent1].[Sid] AS [Sid],

    [Extent1].[SchoolName] AS [SchoolName]

    FROM [dbo].[SchoolInfo] AS [Extent1]

这次却是正常的

现在查询ClassInfo表:

var list = db.QueryClass.ToList();

生成sql语句如下:(问题2)

SELECT

    [Extent1].[Id] AS [Id],

    [Extent1].[SchoolName] AS [SchoolName],

    [Extent1].[ClassName] AS [ClassName],

    [Extent1].[SchoolId] AS [SchoolId]

    FROM [dbo].[ClassInfo] AS [Extent1]

可以看到SQL语句并未像SchoolInfo表一样出现表联查语句

疑问:

HasRequired(x => x.School).WithRequiredDependent(x => x.ClassInfo).Map(map => map.MapKey("SchoolId")).WillCascadeOnDelete(true);

ClassInfo必须有SchoolInfo,SchoolInfo又依赖ClassInfo,二者是生死存亡的关系,缺一不可,可是这样无法解释问题1和问题2

当关系为单向一对一关系时查询是正常的,不会出现表联查现象

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