private Flowable<WriteResult> deleteByKey(StandardOperations.DeleteByKey op) {
      ObjectNode json = objectMapper.createObjectNode();
      ObjectNode query = QueryBuilders.idsQuery(op.keys()).toJson(objectMapper);
      json.set("query", query);
      return ops.deleteByQuery(json).toFlowable();
    }
Queries <a href="https://www.elastic.co/">ElasticSearch</a> data-store.
criteria/elasticsearch/src/org/immutables/criteria/elasticsearch/ElasticsearchBackend.java