用于从另一个模型（Django）获取列的外键

我一直在尝试向我的培训主题模型添加一个名为“email”的新列，并通过外键将其值链接到另一个模型（培训），然后在前端显示所有内容（后面是django，前面是react）

我已将此行添加到培训主题模型中：

email = models.CharField(_('Email assigned'), max_length=100, default='', blank=True)

email_to = models.ForeignKey(TrainingSubject, related_name='manager_email', default='', blank=True)  

而这一款则适用于培训车型：

email = models.CharField(_('Email assigned'), max_length=100, default='', blank=True)

email_to = models.ForeignKey(TrainingSubject, related_name='manager_email', default='', blank=True)  

全文如下：

    class TrainingSubject(models.Model):
    """TrainingSubject model for storing subjects."""

    name = models.CharField(_('name'), max_length=160, unique=True)
    email = models.CharField(_('Email assigned'), max_length=100, default='', blank=True)

    def __str__(self):
        return self.name

    class Meta:
        verbose_name = _('Training subject')
        verbose_name_plural = _('Training subjects')


class Training(models.Model):
    """Training model for storing trainings."""

    subject = models.ForeignKey(TrainingSubject, verbose_name=_('subject'),
                                related_name='trainings')
    email_to = models.ForeignKey(TrainingSubject, related_name='manager_email', default='', blank=True)                                
    date = models.DateTimeField(_('date'))
    timezone = models.CharField(

迁移无法应用，每次我开始创建这些东西时都会出现此错误

        Running migrations:
      Rendering model states... DONE
      Applying sonel_core.0089_training_email_to...Traceback (most recent call last):
      File "/home/vagrant/.pyenv/versions/cms/lib/python3.5/site-packages/django/db/backends/utils.py", line 64, in execute
        return self.cursor.execute(sql, params)
    psycopg2.IntegrityError: column "email_to_id" contains null values

以前我尝试只在一个步骤中添加所有内容，现在我将其分解为两个步骤：1.将电子邮件添加到培训主题（工作非常好）和2.将电子邮件添加到培训模型中，并最终能够在前端显示此电子邮件

---

我做错了什么？

定义字段以接受空值，因为表已经有一些数据，添加新FK字段时，必须提供空值或有效值

email_to = models.ForeignKey(TrainingSubject, related_name='manager_email', default='', blank=True, null=True)