c# – 实体框架：在WPF中创建和更新相关对象

我正在使用EF 6.0和代码优先方法.
我有通过Entity Framework在db中创建和更新数据的问题.我不确定在存储Student之前是否需要创建db.Groups.Attach(student.Group).保存学生后没有这个,我也有新的组,其名称与其他GroupId相同.

此外,我无法更新学生,因为我遇到异常：无法定义两个对象之间的关系,因为它们附加到不同的ObjectContext对象.

public class Student {
    [Key]
    public int StudentId {get; set;}
    public string Name {get; set;}
    public Group Group {get; set;}
}
public class Group {
    [Key]
    public int GroupId{ get; set;}
    public string Name {get; set;}
    public virtual ICollection<Student> Students {get; set;}
}

.

public class StudentDao {
    public void createStudent(Student student) {
        using (var db = new StorageContext()) {
            // without this also creates new Group.
            db.Groups.Attach(student.Group);    
            db.Students.Add(student);
            db.SaveChanges();
        }
    }

    public void updateStudent(Student student) {
        using (var db = new StorageContext()) {
            var original = db.Students.Find(student.StudentId);
            if (original != null) {
                original.Name = student.Name;
                original.Group =  student.Group;
                db.SaveChanges();   //exception
            }
        }
    }
}