导航属性返回插入空后，

问题描述

我已经迁移从EF4我的应用程序EF5。我用下面的code与previous版本，以获得一个新增加的项目相关的实体。

I've migrated my application from EF4 to EF5.I used the below code with the previous version to get related entity of a newly added item.

Student s = new Student();
s.Name = _name;
s.ClassID = _cID;

db.Students.Add(s);
db.SaveChanges();

ClassRoom c = s.ClassRoom;

让我用来获取特定的类实体 C 。但现在 s.ClassRoom 返回null。

so I used to get the specific class entity to c. But now s.ClassRoom returns null.

我如何获得学生的教室实体？我一定要使用 db.ClassRooms.FirstOrDefault（....）？

How do I get the ClassRoom entity for the student? Do I have to use db.ClassRooms.FirstOrDefault(....)?

推荐答案

问题是，你有没有装导航属性呢。

The issue is that you have not loaded the navigation property yet.

您可以使用：

db.Students.Include("ClassRoom")

或

using System.Data.Entity;
db.Students.Include(s=>s.ClassRoom)

要急切地加载导航属性。

to eagerly load the nav property

另外一种选择是通过标记与虚拟导航属性启用延迟加载。我个人preFER前（热切加载），因为它鼓励更多的高性能code。

the other option is to enable lazy loading by marking the navigation property with virtual. I personally prefer the former (eager loading) as it encourages more performant code.

另外这里看看我的导航性能的文章中，我谈到加载启动<近href="http://blog.staticvoid.co.nz/2012/7/17/entity_framework-navigation_property_basics_with_$c$c_first" rel="nofollow">http://blog.staticvoid.co.nz/2012/7/17/entity_framework-navigation_property_basics_with_$c$c_first

Also check out my navigation properties article here, I talk about loading near the start http://blog.staticvoid.co.nz/2012/7/17/entity_framework-navigation_property_basics_with_code_first

您code应改为：

Student s = new Student();
s.Name = _name;
s.ClassID = _cID;

db.Students.Add(s);
db.SaveChanges();

//reload the entity from the DB with its associated nav property
s = db.Students.Include(s=>s.ClassRoom).Single(st=>st.StudentId == s.StudentId);
ClassRoom c = s.ClassRoom;

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