__str__-error.py

# models.py

class State(models.Model):
    name = models.CharField(_('nome'), max_length=60)

    class Meta:
        verbose_name = _('estado')
        verbose_name_plural = _('estados')

    def __str__(self):
        return self.name


class City(models.Model):
    name = models.CharField(_('nome'), max_length=60)
    state = models.ForeignKey('regions.State', verbose_name=_('estado'), related_name='cities')

    class Meta:
        verbose_name = _('cidade')
        verbose_name_plural = _('cidades')

    def __str__(self):
        return self.name + ' ' + self.state.name
    
# forms.py

class IssuerForm(forms.ModelForm):
    class Meta:
        model = Issuer
        fields = [
            'name',
            'cnpj',
            'city' # fk para City
        ]