Fix bug in NestedUtils.partitionByChildren()

[romseygeek]

If multiple fields appeared between two child scopes, the following children
would be incorrectly assigned to the parent scope.

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)

[cbuescher]

Thanks, I left a comment to further simplify the new FieldFetcherTest and maybe add assertions on the whole output there now that its smaller.

[cbuescher]

I think this boils it down even further to make the test more succinct. At least this fails without your above changes and passes otherwise:

        MapperService mapperService = createMapperService("""
            { "_doc" : { "properties": {
                                 "ml": {
                                   "properties": {
                                     "feature_importance": {
                                       "type": "nested",
                                       "dynamic": "false",
                                       "properties": {
                                         "feature_name": {
                                           "type": "keyword"
                                         }
                                       }
                                     },
                                     "is_training": {
                                       "type": "boolean"
                                     },
                                     "prediction_probability": {
                                       "type": "double"
                                     },
                                     "top_classes": {
                                       "type": "nested",
                                       "properties": {
                                         "class_name": {
                                           "type": "keyword"
                                         }
                                       }
                                     }
                                   }
                                 }
            }}}
            """);

        String source = """
            {
                       "ml": {
                         "y_prediction": "no",
                         "top_classes": [
                           {
                             "class_name": "no"
                           },
                           {
                             "class_name": "yes"
                           }
                         ],
                         "prediction_probability": 0.978734716971892,
                         "is_training": false
                       }
                     }
            """;

[cbuescher]

Maybe if that version is working for you, could you also change the test slightly so we don't just asset on the substring but actually check the whole output map? Shouldn't be too deep any more now and makes the assertion.

[romseygeek]

I think it's important to have more than one entry before and between the nested mappers, to cover all the cases properly? So yes, the cut down version will cover the specific bug here but I'd like to keep more in there to catch regressions.

[elasticsearchmachine]

💔 Backport failed

Status	Branch	Result
✅	8.9
❌	7.17	Commit could not be cherrypicked due to conflicts

You can use sqren/backport to manually backport by running backport --upstream elastic/elasticsearch --pr 97970