.net - Entity Framework 5 中相关实体集合的问题

Question

我的项目中有以下四个实体。

学生

public int StudentID { get; set; }
public string StudentName { get; set; }
public int StandardId { get; set; }

public virtual Standard Standard { get; set; }
public virtual StudentAddress StudentAddress { get; set; }
public virtual ICollection<Course> Courses { get; set; }

老师

public int TeacherId { get; set; }
public string TeacherName { get; set; }
public Nullable<int> StandardId { get; set; }

public virtual ICollection<Course> Courses { get; set; }
public virtual Standard Standard { get; set; }

标准

public int StandardId { get; set; }
public string StandardName { get; set; }
public string Description { get; set; }

public virtual ICollection<Student> Students { get; set; }
public virtual ICollection<Teacher> Teachers { get; set; }

课程

public int CourseId { get; set; }
public string CourseName { get; set; }
public string Location { get; set; }
public int TeacherId { get; set; }

public virtual Teacher Teacher { get; set; }
public virtual ICollection<Student> Students { get; set; }

如果我尝试直接从学生实体访问课程列表，我总是将课程计数为零。

   using (SchoolDBEntities db = new SchoolDBEntities())
    {
        var query = (from s in db.Students
                     select s).FirstOrDefault<Student>();

        foreach (var item in query.Courses)
        {
                Console.WriteLine(item.CourseName);                    
        }
        Console.ReadLine();
    }

但是如果我访问下面的课程列表，我会正确获取数据，第一种方法有什么问题？我错过了什么吗？我希望下面的方法不是访问数据的有效方法。

    using (SchoolDBEntities db = new SchoolDBEntities())
    {
        var query =   (from s in db.Students                               
                         select s).FirstOrDefault<Student>();

        foreach (var item in query.Standard.Teachers)
        {
            foreach (var crs in item.Courses)
            {
                Console.WriteLine(crs.CourseName);
            }
        }                
        Console.ReadLine();
    }