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
映射关系为：
再建一个数据访问类：
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