ForeignKey in a Model form with "add new" button

RelatedFieldWidgetCanAdd.py

class OrderNewForm(forms.ModelForm):

   client = forms.ModelChoiceField(
       required=False,
       queryset=Client.objects.all(),
       widget=RelatedFieldWidgetCanAdd(Client, related_url="so_client_add")
                                )
   class Meta:
       model = Order
       fields = ('code', 'client')


class RelatedFieldWidgetCanAdd(widgets.Select):

   def __init__(self, related_model, related_url=None, *args, **kw):

       super(RelatedFieldWidgetCanAdd, self).__init__(*args, **kw)

       if not related_url:
           rel_to = related_model
           info = (rel_to._meta.app_label, rel_to._meta.object_name.lower())
           related_url = 'admin:%s_%s_add' % info

       # Be careful that here "reverse" is not allowed
       self.related_url = related_url

   def render(self, name, value, *args, **kwargs):
       self.related_url = reverse(self.related_url)
       output = [super(RelatedFieldWidgetCanAdd, self).render(name, value, *args, **kwargs)]
       output.append('<a href="%s" class="add-another" id="add_id_%s" onclick="return showAddAnotherPopup(this);"> ' % \
           (self.related_url, name))
       output.append('<img src="%sadmin/img/icon_addlink.gif" width="10" height="10" alt="%s"/></a>' % (settings.STATIC_URL, 'Add Another'))
       return mark_safe(''.join(output))

Credits to: https://stackoverflow.com/questions/28068168/django-adding-an-add-new-button-for-a-foreignkey-in-a-modelform