return true if at least one book has
           file with the given url as attachment

def file_is_attached(self, url):
        '''return true if at least one book has
           file with the given url as attachment
        '''
        body = self._get_search_field('_attachments.url', url)
        return self.es.count(index=self.index_name, body=body)['count'] > 0