title: Dynamically set choices in django model based on whether adding a new record or editing existing
body: <p>I'd like to have one set of choices if adding a new record and a different set if editing an existing record. I thought something like the following would work but it doesn't.</p>
  
  <pre><code>class MyModel(models.Model):
      CHOICES = (('1', 'Option 1'), ('2', 'Option 2'), ('3', 'Option 3'), ('4', 'Option 4'),)
      def test(self):
          if self.pk:
              return True
      if test is True:
          CHOICES = (('1', 'Option 1'), ('2', 'Option 2'),)
      my_field = models.CharField(max_length=1, choices=CHOICES)
  </code></pre>