您可以覆盖 save $您的 modelB 模型的c $ c>方法，而不是提供默认值：

You can override the save method of your modelB model instead of providing a default value:

class ModelB(models.Model):
    modela = models.OneToOneField(ModelA)
    b_b = models.CharField(max_length=6, choices=[(1, 'M'), (2, 'F')], blank=True)

    def save(self, *args, **kwargs):
        if not self.b_b:
            self.b_b = self.modela.a_b
        super(ModelB, self).save(*args, **kwargs)

在评论中澄清后编辑

如果您只想向参观者显示价值，但想保留事实上， ModelB.b_b 是空的，逻辑应该放在模板中。

Edit after clarification in the comments

If you just want to display the value to visitors but want to keep the fact that the ModelB.b_b is empty, the logic should lay in the template.

假设你的视图集 objB 在上下文中，您有兴趣显示的 ModelB 对象，相关模板的一部分应该如下所示：

Assuming your view set objB in the context being the ModelB object your are interested to display, the relevant part of the template should look like:

{% with objB.b_b as bValue %}
{% if not bValue %}
    <p>{{ objB.modela.a_b }}</p>
{% else %}
    <p>{{ bValue }}</p>
{% endif %}
{% endwith %}