Historically projections on multi-valued have not been possible, see HSEARCH-1895 Open in particular. The exact reasons are a bit unclear.

It would be interesting to support them, especially in combination with the nested object field support.

Note that two cases, at least, should be considered:

1. Multi-valued fields on the root document
2. Single-valued fields in multi-valued nested documents

APIs and implementations are still TBD, but we'll most likely leverage the projection DSL ( HSEARCH-3222 In Progress ), in particular the composite projections ( HSEARCH-3390 Open ). We may have to rely on metadata telling us which field is potentially multi-valued and which is not, see HSEARCH-3324 Open .