c# EntityFramework 5在插入时丢失引用

我有EF5，并希望(有…)建立一个多层应用程序。EF是我的持久层，我为我的客户端使用WCF。现在我的业务逻辑有一个问题，我不太明白。

我有一个方法应该保留一些东西。当我使用. savechanges()将Reservation发送到数据库时，一切都OK了。

public bool bReservieren(Student oStudent, Korb oKorb, IEnumerable<Service> lServices, DateTime dtFrom, DateTime dtTo)
    {
        try
        {
            BeachChaireModelContainer context = new BeachChaireModelContainer();
            Reservierung newReservation = context.Reservation.Create();
            //Tried it with newReservation.Student = oStudent; But there is an 
            //error cause of the different context´s so I search the correct Student
            //again. (same with Korb)
            newReservation.Student = context.Student.Find(oStudent.Kartennummer);
            newReservierung.Korb = context.Korb.Find(oKorb.KID);
            newReservierung.dtVON = dtFrom;
            newReservierung.dtBIS = dtTo;
            newReservierung = context.Reservation.Add(newReservation);
               //This if isnt relevant yet. lService is null in my tests     
               if (lServices != null)
               {
                  foreach (var item in lServices)
                  {
                     newReservation.Service.Add(context.Service.Find(item.SeID));
                  }
               }
               context.SaveChanges();
               return true;
        }
        catch (Exception)
        { }
        return false;
    }

因此，在我执行此命令并调试它之后，将创建一个包含所有正确引用的新保留!但是当我再次尝试使用以下命令获取Reservation时:

//had to mention it is a new Methode with a new context, but this should
//be no Problem in my oppinion? 
var query = from it in context.Reservation
            select it;

我有学生和Korb属性设置为空值?在数据库中一切都是正确的。(存储正确的外键)

有人知道这种行为吗?

换句话说，当我试图再次使用另一个上下文获取Reservation时，我无法执行以下操作:

String test = newReservation.Student.Surename;

原因Student == null

var query = from it in context.Reservation.Include("Student")
            select it;