title: Make a field required if another field is checked in Django form
body: <p>I have a model like this:</p>
  
  <pre><code>class News(models.Model):
      is_activity = models.BooleanField(default=False)
      activity_name = models.CharField(max_length=240, blank=True, null=True)
  </code></pre>
  
  <p>What I am trying to achieve is, if <code>is_activity</code> is checked in I want <code>activity_name</code> to be required. Thus, I am trying to override the <code>__init__</code> method:</p>
  
  <pre><code>class NewsForm(forms.ModelForm):
      class Meta:
          model = News
  
      def __init__(self, *args, **kwargs):
          super(NewsForm, self).__init__(*args, **kwargs)
          if self.fields['is_activity'] is True:
              self.fields['activity_name'].required = True
  
  
  class NewsAdmin(FrontendEditableAdminMixin, admin.ModelAdmin):
      form = NewsForm
  </code></pre>
  
  <p>Even if I check in the <code>is_activity</code> the <code>activity_name</code> is non-required. What's wrong?</p>