Changed inner_hits to work with the new join field type and

[martijnvg]

at the same time maintaining support for the _parent meta field type.

Relates to #20257

[jimczi]

Thank you for rewriting all the tests.
I left one comment. I think we need to be explicit when we build the inner hit query.

[jimczi]

I think we can remove the leniency and make sure that we build parent or children inner hits depending on the context. hasParent should only build inner hits of parent documents and hasChild for children documents.
With this information in JoinFieldInnerHitSubContext you can just call:

joinFieldMapper.getJoinFieldMapper(typeName, isParentInnerHits);

and use JoinFieldMapper#hasParent and JoinFieldMapper#hasChild to build the inner hits query ?

[jimczi]

Does this work if the document is a child and a parent ? I think it's simpler if we use the context directly.
hasParent expects a child and builds parent inner hits. hasChildren expects a parent and builds children inner hits ?

[martijnvg]

@jimczi I made the join field inner hits logic more explicit.

[jimczi]

LGTM

[Yneth]

My mapping
{ "mappings": { "test": { "properties": { "views": { "type": "nested", "properties": { "date": { "type": "date" } } } } } } }

My query to sort by view count
{ "query": { "has_child": { "type": "views", "score_mode": "sum", "query": {"match_all": {}}, // here I would also like to add some filtering by date "inner_hits": {} } } }

throws
{ "error": { "root_cause": [ { "type": "query_shard_exception", "reason": "[has_child] no join field has been configured", "index_uuid": "XW9AAGtjRtutQ3n4mhZ9zQ", "index": "test" } ], "type": "search_phase_execution_exception", "reason": "all shards failed", "phase": "query", "grouped": true, "failed_shards": [ { "shard": 0, "index": "test", "node": "_bbwp3g_TfOGbZE4McKnjw", "reason": { "type": "query_shard_exception", "reason": "[has_child] no join field has been configured", "index_uuid": "XW9AAGtjRtutQ3n4mhZ9zQ", "index": "test" } } ] }, "status": 400 }