Django ValueError： 无法解析相关模型 u'app.model'

我有两个应用程序(比如ook和eek)，我想使用eek中的模型中ook中的模型的外键.这两个都是INSTALLED_APPS分，ook分是第一名.

在ook.models.py年里，我有:

class Fubar(models.Model):
    ...

在eek.models.py年里，我有:

class monkey(models.Model):
    external = models.ForeignKey('ook.Fubar', blank=True, null=True)
    ...

生成的迁移是:

class Migration(migrations.Migration):

    dependencies = [
        ('eek', '0002_auto_20151029_1040'),
    ]

    operations = [
        migrations.AlterField(
            model_name='monkey',
            name='external',
            field=models.ForeignKey(blank=True, to='ook.Fubar', null=True),
        ),
    ]

当我运行迁移时，我收到以下错误:

   ...
   1595             raise ValueError('Foreign Object from and to fields must be
the same non-zero length')
   1596         if isinstance(self.rel.to, six.string_types):
-> 1597             raise ValueError('Related model %r cannot be resolved' % self.rel.to)
   1598         related_fields = []
   1599         for index in range(len(self.from_fields)):
ValueError: Related model u'ook.Fubar' cannot be resolved

What am I doing wrong?