Django models的詭異異常RelatedObjectDoesNotExist

models代碼如下：

class Course(models.Model):
    name = models.CharField(unique=True, max_length=64)
    price = models.PositiveIntegerField(default=19800)
    outline = models.TextField()

    def __str__(self):
            return "%s" % self.name

    class Meta:
        verbose_name_plural = "課程信息表"
        verbose_name = "課程信息表"


class ClassList(models.Model):
    course = models.ForeignKey("Course")
    semester = models.PositiveIntegerField(verbose_name="學期")
    class_type_choice = ((0, '脫產'),
                  (1, '周末'),
                  (2, '網絡'),
                  )
    branch = models.ForeignKey("Branch")
    class_type = models.PositiveIntegerField(choices=class_type_choice)
    teachers = models.ManyToManyField("UserProfile")
    start_date = models.DateField()
    end_date = models.DateField()

    def __str__(self):
            return "%s(%s)" % (self.course, self.semester)

    class Meta:
        verbose_name_plural = "班級信息表"
        verbose_name = "班級信息表"
        unique_together = ("course", "semester")

兩個表關系如上，ClassList表的course是一個外鍵，關聯上面的Course表。

我想在ClassList中定義__str__函數，就像代碼中寫的。

我是用django的modelform來處理數據並構建web頁面。

結果在進入那個由modelform構建的頁面時，發生了異常

 

 

 C:\Users\Administrator\AppData\Roaming\Python\Python35\site-packages\django\db\models\fields\related_descriptors.py 下的line 193

        if rel_obj is None and not self.field.null:
            raise self.RelatedObjectDoesNotExist(
                "%s has no %s." % (self.field.model.__name__, self.field.name)
            )
        else:
            return rel_obj

so，就是那個RelatedObjectDoesNotExist，google了很久，發現有個網友說的很好：

https://stackoverflow.com/questions/26270042/how-do-you-catch-this-exception#comment41357708_26270785

 

那個票最多的答案。

 

 正如他所說的Django is amazing when its not terrifying.我此時的內心是崩潰的。。。

代碼更改如下:

class ClassList(models.Model):
    course = models.ForeignKey("Course")
    semester = models.PositiveIntegerField(verbose_name="學期")
    class_type_choice = ((0, '脫產'),
                  (1, '周末'),
                  (2, '網絡'),
                  )
    branch = models.ForeignKey("Branch")
    class_type = models.PositiveIntegerField(choices=class_type_choice)
    teachers = models.ManyToManyField("UserProfile")
    start_date = models.DateField()
    end_date = models.DateField()

    def __str__(self):
        try:
            return "%s(%s)" % (self.course, self.semester)
        except Course.DoesNotExist:
            return "%s" % self.semester

 

正如大神網友所說，我抓住了這個異常，隨便返回個不是外鍵的值就好了。

無FUCK說~~~希望能幫到大家。。。