Django自定义模型字段在admin中被多次调用

以下是我正在创建的自定义字段：

class TransactionStateField(with_metaclass(models.SubfieldBase, models.CharField)):

    def __init__(self, *args, **kwargs):
        kwargs['max_length'] = 255
        super(TransactionStateField, self).__init__(*args, **kwargs)

    def to_python(self, value):
        if value:
            print value.__class__, 11111111111
            kls = import_by_path(value)
            return kls()
        return None

    def get_prep_value(self, value):
        if value:
            return fullname(value)
        return None

这就是模型：

class AbstractTransaction(models.Model):
    processed_at = models.DateTimeField(null=True, blank=True)
    cancelled_at = models.DateTimeField(null=True, blank=True)
    created_at = models.DateTimeField(auto_now_add=True)
    amount = models.DecimalField(max_digits=10, decimal_places=2)

    class Meta:
        abstract = True

    def process(self):
        raise NotImplemented()


class Transaction(AbstractTransaction):
    CURRENCIES = (('NZD', 'New Zealand Dollars'), ('EUR', 'Euro'))
    currency = models.CharField(choices=CURRENCIES, max_length=25)
    user_goid = models.CharField(null=True, blank=True, max_length=255)
    order_goid = models.CharField(null=True, blank=True, max_length=255)
    billingtoken = models.ForeignKey('BillingToken', null=True, blank=True)
    state = TransactionStateField(default='project.apps.transactions.states.NewTransaction')

    def process(self):
        self.state.process(self)

    def __unicode__(self):
        return "{} ${}".format(self.currency, self.amount)

这是管理员：

class SubTransactionInline(admin.StackedInline):
    model = SubTransaction
    extra = 0


class TransactionAdmin(admin.ModelAdmin):
    inlines = [SubTransactionInline]

admin.site.register(Transaction, TransactionAdmin)

当我在admin中创建事务记录（内联中没有子事务）时，

to_python

方法会被调用3次，调用的值类型不同

<type 'unicode'> 11111111111
<type 'unicode'> 11111111111
<class 'project.apps.transactions.states.NewTransaction'> 11111111111

>>> t = Transaction.objects.create(amount=11111, currency='NZD')
<type 'unicode'> 11111111111