docs: def elastic(self, resource=None):
          """Get ElasticSearch instance for given resource."""
          px = self._resource_prefix(resource)
  
          if px not in self.elastics:
              url = self._resource_config(resource, 'URL')
              assert url, 'no url for %s' % px
              self.elastics[px] = get_es(url, **self.kwargs)
  
          return self.elastics[px]