Add FlatObject FieldMapper

CHANGELOG.md

@@ -107,6 +107,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Changed
 - Require MediaType in Strings.toString API ([#6009](https:/opensearch-project/OpenSearch/pull/6009))
 - [Refactor] XContent base classes from xcontent to core library ([#5902](https:/opensearch-project/OpenSearch/pull/5902))
+- Added a new field type: flat_object ([#6507](https:/opensearch-project/OpenSearch/pull/6507))
 
 ### Deprecated
 - Map, List, and Set in org.opensearch.common.collect ([#6609](https:/opensearch-project/OpenSearch/pull/6609))

modules/lang-painless/src/yamlRestTest/resources/rest-api-spec/test/painless/30_search.yml

@@ -482,3 +482,89 @@
  }]
  - match: { error.root_cause.0.type: "illegal_argument_exception" }
  - match: { error.root_cause.0.reason: "script score function must not produce negative scores, but got: [-9.0]"}
+
+---
+
+"Flat-object fields from within the scripting":
+ - skip:
+ version: " - 2.99.99"
+ reason: "flat_object is introduced in 3.0.0 in main branch"
+
+ - do:
+ indices.create:
+ index: test
+ body:
+ mappings:
+ properties:
+ flat:
+ type : "flat_object"
+
+ - do:
+ index:
+ index: test
+ id: 1
+ body: {
+ "flat": {
+ "field_1": "John Doe",
+ "field_2": 33,
+ "field_3": false
+ }
+ }
+
+ - do:
+ index:
+ index: test
+ id: 2
+ body: {
+ "flat": {
+ "field_1": "Joe Public",
+ "field_2": 45
+ }
+ }
+
+ - do:
+ indices.refresh:
+ index: test
+
+ # It is possible to filter based on the number of fields in the flat-object
+ - do:
+ search:
+ body: {
+ _source: true,
+ query: {
+ bool: {
+ filter: {
+ script: {
+ script: {
+ source: "doc['flat'].size() > 2",
+ lang: "painless"
+ }
+ }
+ }
+ }
+ }
+ }
+
+ - length: { hits.hits: 1 }
+ - match: { hits.hits.0._source.flat.field_1: "John Doe" }
+
+ - do:
+ search:
+ body: {
+ _source: true,
+ query: {
+ bool: {
+ filter: {
+ script: {
+ script: {
+ source: "doc['flat'].size() < 3",
+ lang: "painless"
+ }
+ }
+ }
+ }
+ }
+ }
+
+ - length: { hits.hits: 1 }
+ - match: { hits.hits.0._source.flat.field_1: "Joe Public" }