docs: def find_one(self, resource, req, **lookup):
          """Find single document, if there is _id in lookup use that, otherwise filter."""
  
          if config.ID_FIELD in lookup:
              return self._find_by_id(resource=resource, _id=lookup[config.ID_FIELD], parent=lookup.get('parent'))
          else:
              args = self._es_args(resource)
              filters = [{'term': {key: val}} for key, val in lookup.items()]
              query = {'query': {'constant_score': {'filter': {'and': filters}}}}
  
              try:
                  args['size'] = 1
                  hits = self.elastic(resource).search(body=query, **args)
                  docs = self._parse_hits(hits, resource)
                  return docs.first()
              except elasticsearch.NotFoundError:
                  return