Python 参考2个表的模型字段

我正在试着定制

我希望它可以为每个问题和每个答案添加图像。但有些人也不会有任何图像

这是实际模型：

class Question(models.Model):

    quiz = models.ManyToManyField(Quiz, blank=True, )

    category = models.ForeignKey(Category, blank=True, null=True, )

    content = models.CharField(max_length=1000, 
                               blank=False, 
                               help_text="Enter the question text that you want displayed",
                               verbose_name='Question',
                               )

    explanation = models.TextField(max_length=2000,
                                   blank=True,
                                   help_text="Explanation to be shown after the question has been answered.",
                                   verbose_name='Explanation',
                               )


    class Meta:
        verbose_name = "Question"
        verbose_name_plural = "Questions"
        ordering = ['category']


    def __unicode__(self):
        return self.content


class Answer(models.Model):
    question = models.ForeignKey(Question)

    content = models.CharField(max_length=1000, 
                               blank=False, 
                               help_text="Enter the answer text that you want displayed",
                               )

    correct = models.BooleanField(blank=False, 
                                  default=False,
                                  help_text="Is this a correct answer?"
                                  )

    def __unicode__(self):
        return self.content

在这里，我得到了这个解决方案：

Create new model with

image,
type - answer or question
answer_id or question_id

但我对如何创造它有些怀疑。到目前为止，我得到的是：

class Image(models.Model):
    TYPE_CHOICES = (
        ('A','Answer'),
        ('Q','Question'),
    )
    image = models.ImageField(upload_to='static/img')
    type = models.charField(max_length=1, choices=TYPE_CHOICES)
    id = ???

    def __unicode__(self):
        return self.type

---

如何定义id字段？如果它应该引用另外两个表（问题或答案），那么我不知道该怎么做。

Django不支持复合主键。检查答案：谢谢，这真的很有帮助