Django:无法分配必须是实例

我想把一个模型的非str对象取到另一个模型中。。。

这是我的模型:

class Selectdatefield(models.Model):
     User       = models.OneToOneField(settings.AUTH_USER_MODEL,related_name="Users",on_delete=models.CASCADE,null=True,blank=True)
     Start_Date = models.DateField(blank=True, null=True)
     End_Date   = models.DateField(blank=True, null=True)

class Journal(models.Model):
     User       = models.ForeignKey(settings.AUTH_USER_MODEL,on_delete=models.CASCADE,null=True,blank=True)
     company    = models.ForeignKey(Company,on_delete=models.CASCADE,null=True,blank=True,related_name='Companyname')
     start_date = models.ForeignKey(Selectdatefield,on_delete=models.CASCADE,null=True,blank=True,related_name='startdate')
     end_date   = models.ForeignKey(Selectdatefield,on_delete=models.CASCADE,null=True,blank=True,related_name='enddate')
     Date       = models.DateField()
     By         = models.ForeignKey(Ledger1,on_delete=models.CASCADE,related_name='Debitledgers')
     To         = models.ForeignKey(Ledger1,on_delete=models.CASCADE,related_name='Creditledgers')
     Debit      = models.DecimalField(max_digits=10,decimal_places=2)
     Credit     = models.DecimalField(max_digits=10,decimal_places=2)

我曾在我的日志创建视图中尝试过:

  class journalCreateView(LoginRequiredMixin,CreateView):
        model = journal
        form_class  = journalForm

        def form_valid(self, form):
              form.instance.User = self.request.user
              c = company.objects.get(pk=self.kwargs['pk'])
              form.instance.Company = c
              s = selectdatefield.objects.get(pk=self.kwargs['pk3'])
              form.instance.start_date = s
              form.instance.end_date = s.End_Date
              return super(journalCreateView, self).form_valid(form)

我想在提交日志表单时自动获取日志模型中selectdate的结束日期。。。

但是得到这个错误:

ValueError: Cannot assign "datetime.date(2018, 10, 6)": "journal.end_date" must be a "selectdatefield" instance.

错误在这行代码中:

form.instance.end_date = s.End_Date

有人能告诉我我的代码有什么不对吗???