Django Override -Formularfelder in Administrator

class CompanyAdmin(admin.ModelAdmin):
    def render_change_form(self, request, context, *args, **kwargs):
         context['adminform'].form.fields['theme'].queryset = Theme.objects.filter(name__iexact='company')
         return super(CompanyAdmin, self).render_change_form(request, context, *args, **kwargs)
mhmoud sabry