To clarify: projecting on a one-way FieldBridge is not the same as projecting on an unknown field. When attempting to project on a field mapped by a one-way FieldBridge we indeed throw an exception, hopefully helpful. AFAIR we allow to project on "unknown fields" treating them as strings though.

Right... Well, then there also is a missing feature on the Elasticsearch side, since we throw exceptions on unknown fields. I created a ticket to align Elasticsearch on Lucene: HSEARCH-2471 In Progress . I'll submit a PR for that one (+ others) soon.