只有在selectbox具有特定值时，django管理显示字段

models.py

models.py

class FixedIncome(Asset):
   isin = models.CharField(max_length=6, blank=False)
   coupon = models.DecimalField(max_digits=7, decimal_places=4, help_text="Percent")
   payment_frequency = models.ForeignKey(PaymentFrequency)
   months = models.ManyToManyField(Month, blank = False, verbose_name = "Payment months")
   duration = models.DecimalField(max_digits=4, decimal_places=2, default='0')
   maturity_date = models.DateTimeField(default=datetime.now, blank=False)

   class Meta:
      verbose_name_plural = "Fixed Income"
class FixedIncome(Asset