9.0.0 - [inner_hits] fields doesn't support values of type: VALUE_STRING

[sergey-tihon]

Elastic.Clients.Elasticsearch version: 9.0.0

Elasticsearch version: 9.0.0

.NET runtime version: 9.0.4

Operating system version: macOS

Description of the problem including expected versus actual behavior:

Incorrect query serialization for inner_hits.fields.
According to spec it can be just string (not an array): https://github.com/elastic/elasticsearch-specification/blob/main/specification/_types/common.ts#L137

but elastic reply with error [inner_hits] fields doesn't support values of type: VALUE_STRING when fields serialized as string instead of arrray.

Steps to reproduce:

Execute query with fields serialized as string

"inner_hits": {
   "fields": "passages.text",
   "name": "elser-chunk",
   "size": 1,
   "_source": false
},

Expected behavior

Server should not return an error in this case, or serializer should always serialize fields as array. Following example work as expected:

"inner_hits": {
   "fields": ["passages.text"],
   "name": "elser-chunk",
   "size": 1,
   "_source": false
},

Provide DebugInformation (if relevant):

// cc @flobernd please take a look, it looks like a significant regression.

[flobernd]

Hi @sergey-tihon, thanks for reporting. Just to clarify. Did that work in ES (server) 8.x?

It seems that either the specification is incorrect or the server behavior has changed and somebody forgot to update the specification as well.

[sergey-tihon]

We did not have this issue before because Elastic.Clients.Elasticsearch 8.17.4 serialized fields as array.

"inner_hits": {
   "fields": ["passages.text"],
   "name": "elser-chunk",
   "size": 1,
   "_source": false
},

I only have access to the cluster with ES (server) 8.18.0 and it behaves in the same way:

• work when serialized as array
• reply with[inner_hits] fields doesn't support values of type: VALUE_STRING error when serialized as string

[flobernd]

Thanks for confirming @sergey-tihon, this definitely is a bug in the specification (it should be Field[] instead of Fields).