Get a document or a page using its ID
        Won't instantiate them if they are not yet available

def get(self, obj_id):
        """
        Get a document or a page using its ID
        Won't instantiate them if they are not yet available
        """
        if BasicPage.PAGE_ID_SEPARATOR in obj_id:
            (docid, page_nb) = obj_id.split(BasicPage.PAGE_ID_SEPARATOR)
            page_nb = int(page_nb)
            return self._docs_by_id[docid].pages[page_nb]
        return self._docs_by_id[obj_id]