Django models的诡异异常RelatedObjectDoesNotExist

models代码如下：

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 = "课程信息表"
        verbose_name = "课程信息表"

class ClassList(models.Model):
    course = models.ForeignKey("Course")
    semester = models.PositiveIntegerField(verbose_name="学期")
    class_type_choice = ((0, '脱产'),
                  (1, '周末'),
                  (2, '网络'),
                  )
    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 = "班级信息表"
        verbose_name = "班级信息表"
        unique_together = ("course", "semester")

两个表关系如上，ClassList表的course是一个外键，关联上面的Course表。

我想在ClassList中定义__str__函数，就像代码中写的。

我是用django的modelform来处理数据并构建web页面。

结果在进入那个由modelform构建的页面时，发生了异常

 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，就是那个RelatedObjectDoesNotExist，google了很久，发现有个网友说的很好：

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

那个票最多的答案。

 正如他所说的Django is amazing when its not terrifying.我此时的内心是崩溃的。。。

代码更改如下:

class ClassList(models.Model):
    course = models.ForeignKey("Course")
    semester = models.PositiveIntegerField(verbose_name="学期")
    class_type_choice = ((0, '脱产'),
                  (1, '周末'),
                  (2, '网络'),
                  )
    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

正如大神网友所说，我抓住了这个异常，随便返回个不是外键的值就好了。

无FUCK说~~~希望能帮到大家。。。