在django中运行syncdb时找不到关系

我在django应用程序的models.py中有这些类。我使用postgresql8.3作为数据库

class Course(models.Model):
    students = models.ManyToManyField(User)
    title = models.CharField(max_length=200)
    description = models.TextField()

    def __unicode__(self):
        return self.title

    class Meta:
        verbose_name_plural="Courses"

class CourseChoicesForm(forms.Form):
    courseoption = forms.ChoiceField(choices=[(x.id,x.title) for x in Course.objects.all()])

当我跑步时 python manage.py syncdb ,我得到一个错误,上面写着 relation course is not found

...
File "/home/me/Django-1.4/django/utils/importlib.py", line 35, in import_module
    __import__(name)
  File "/home/me/dev/python/django/myprj/myapp/models.py", line 50, in <module>
    class CourseChoicesForm(forms.Form):
  File "/home/me/dev/python/django/myprj/myapp/models.py", line 52, in CourseChoicesForm
    courseoption = forms.ChoiceField(choices=[(x.id,x.title) for x in Course.objects.all()])
...
File "/home/me/Django-1.4/django/db/backends/postgresql_psycopg2/base.py", line 52, in execute
    return self.cursor.execute(query, args)
django.db.utils.DatabaseError: relation "myapp_course" does not exist

更新: 我解决这个问题如下

class CourseChoicesForm(forms.Form):
    courseoption = forms.ChoiceField(choices=[],required=False)
    def __init__(self, *args, **kwargs):
        super(CourseChoicesForm, self).__init__(*args, **kwargs)
        self.fields['courseoption'].choices = [(x.id,x.title) for x in Course.objects.all()]

尽管如此,我还是不太清楚为什么会发生这种行为。有人能解释一下吗?