title: django how to create field dependent field
body: <p>I have a post model in which i have post type field.
  I want that when user select post type = assignment this it ask for submission deadline other wise it does not ask anything.
  and how can i display it in template.</p>
  
  <blockquote>
    <p>models.py</p>
  </blockquote>
  
  <pre><code>class Post(models.Model):
      user = models.ForeignKey(settings.AUTH_USER_MODEL, default=1)
      title = models.CharField(max_length=120)
      slug = models.SlugField(unique=True, blank = True)
      content = models.TextField()
      choice = (
          ('post','post'),
          ('anouncement','anouncement'),
          ('question', 'question'),
          ('assignment', 'assignment')
          )
      post_type = models.CharField(choices = choice, default = 'post', max_length = 12)
      classroom = models.ForeignKey(Classroom)
      updated = models.DateTimeField(auto_now=True, auto_now_add=False)
      timestamp = models.DateTimeField(auto_now=False, auto_now_add=True)
  
  
  
      def __unicode__(self):
          return self.title
  
      def __str__(self):
          return self.title
  
      @property
      def comments(self):
          instance = self
          qs = Comment.objects.filter_by_instance(instance)
          return qs
  
      @property
      def get_content_type(self):
          instance = self
          content_type = ContentType.objects.get_for_model(instance.__class__)
          return content_type
  
      def get_absolute_url(self):
          return reverse("posts:detail", kwargs={"slug": self.slug})
  </code></pre>