title: django admin show field only if selectbox has a particular value
body: <p>models.py</p>
  
  <pre><code>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"
  </code></pre>
  
  <p>I want to django admin show the maturity_date attribute, only if month (numeric 1 to 12, selectbox) is selected. I've read something about js, but I can't do it :(</p>