'receives a page_id in value and returns the site_id and page_id
of that page or the current site_id and None if no page_id is given.'

def decompress(self, value):

    if value:
        page = Page.objects.get(pk=value)
        site = page.site
        return [site.pk, page.pk, page.pk]
    site = Site.objects.get_current()
    return [site.pk, None, None]
