django：使用自然外键加载fixture无法使用'ValueError：int（无效的文字，基数为10）

My models are ...

我的模特是......

class StateManager(models.Manager):
    def get_by_natural_key(self, name):
        return self.get(name=name)


class DistrictManager(models.Manager):
    def get_by_natural_key(self, name, state):
        return self.get(name=name, state=state)

class State(models.Model):

    class Meta:
        verbose_name = "State"
        verbose_name_plural = "States"
        permissions = (
            ('access_state', 'Can access States'),
        )

    COUNTRIES = (
        ('India', 'India'),
        ('USA', 'USA'),
        ('Thailand', 'Thailand'),
    )

    # Managers
    objects = StateManager()

    # Database fields
    name = models.CharField(
        'Name',
        max_length=100,
        unique=True,
        help_text='''
        100 chars max
        '''
    )
    code = models.CharField(
        'Code',
        max_length=10,
        unique=True,
        help_text='''
        10 chars max
        ''',
        null=True, blank=True
    )
    country = models.CharField(
        max_length=50,
        default="India",
        choices=COUNTRIES,
        blank=False,
        null=False
    )

    def __str__(self):
        return self.name

    def natural_key(self):
        return (self.name,)


class District(models.Model):

    class Meta:
        verbose_name = "District"
        verbose_name_plural = "Districts"
        unique_together = (
            (
                "state",
                "name"
            ),
        )

    # Managers
    objects = DistrictManager()

    # Database fields
    name = models.CharField(
        'Name',
        max_length=100,
        help_text='''
        100 chars max
        '''
    )
    code = models.CharField(
        'Code',
        max_length=10,
        help_text='''
        10 chars max
        ''',
        null=True, blank=True
    )
    state = models.ForeignKey(State)

    def __str__(self):
        return self.name

    def natural_key(self):
        return (self.name,) + self.state.natural_key()
    natural_key.dependencies = ['parties.state']
class StateMa