[Backport 2.x] Fix search_as_you_type not supporting multi-fields

CHANGELOG.md

@@ -34,6 +34,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Fixed
 - Fix wildcard query containing escaped character ([#15737](https://github.com/opensearch-project/OpenSearch/pull/15737))
 - Add validation for the search backpressure cancellation settings ([#15501](https://github.com/opensearch-project/OpenSearch/pull/15501))
+- Fix search_as_you_type not supporting multi-fields ([#15988](https://github.com/opensearch-project/OpenSearch/pull/15988))
 - Avoid infinite loop when `flat_object` field contains invalid token ([#15985](https://github.com/opensearch-project/OpenSearch/pull/15985))
 
 ### Security

modules/mapper-extras/src/main/java/org/opensearch/index/mapper/SearchAsYouTypeFieldMapper.java

@@ -264,7 +264,15 @@ public SearchAsYouTypeFieldMapper build(Mapper.BuilderContext context) {
             }
             ft.setPrefixField(prefixFieldType);
             ft.setShingleFields(shingleFieldTypes);
-            return new SearchAsYouTypeFieldMapper(name, ft, copyTo.build(), prefixFieldMapper, shingleFieldMappers, this);
+            return new SearchAsYouTypeFieldMapper(
+                name,
+                ft,
+                multiFieldsBuilder.build(this, context),
+                copyTo.build(),
+                prefixFieldMapper,
+                shingleFieldMappers,
+                this
+            );
         }
     }
 
@@ -623,12 +631,13 @@ public SpanQuery spanPrefixQuery(String value, SpanMultiTermQueryWrapper.SpanRew
     public SearchAsYouTypeFieldMapper(
         String simpleName,
         SearchAsYouTypeFieldType mappedFieldType,
+        MultiFields multiFields,
         CopyTo copyTo,
         PrefixFieldMapper prefixField,
         ShingleFieldMapper[] shingleFields,
         Builder builder
     ) {
-        super(simpleName, mappedFieldType, MultiFields.empty(), copyTo);
+        super(simpleName, mappedFieldType, multiFields, copyTo);
         this.prefixField = prefixField;
         this.shingleFields = shingleFields;
         this.maxShingleSize = builder.maxShingleSize.getValue();

modules/mapper-extras/src/test/java/org/opensearch/index/mapper/SearchAsYouTypeFieldMapperTests.java

@@ -298,6 +298,20 @@ private void assertMultiField(int shingleSize) throws IOException {
         }
     }
 
+    public void testSubField() throws IOException {
+        MapperService mapperService = createMapperService(
+            fieldMapping(
+                b -> b.field("type", "search_as_you_type")
+                    .startObject("fields")
+                    .startObject("subField")
+                    .field("type", "keyword")
+                    .endObject()
+                    .endObject()
+            )
+        );
+        assertThat(mapperService.fieldType("field.subField"), instanceOf(KeywordFieldMapper.KeywordFieldType.class));
+    }
+
     public void testIndexOptions() throws IOException {
         DocumentMapper mapper = createDocumentMapper(
             fieldMapping(b -> b.field("type", "search_as_you_type").field("index_options", "offsets"))

rest-api-spec/src/main/resources/rest-api-spec/test/search/390_search_as_you_type.yml

@@ -0,0 +1,75 @@
+setup:
+  - do:
+      indices.create:
+        index:  test_1
+        body:
+          mappings:
+            properties:
+              text:
+                type: search_as_you_type
+                fields:
+                  subField:
+                    type: keyword
+  - do:
+      index:
+        index:  test_1
+        id:     1
+        body:   { text: test search as you type }
+
+  - do:
+      indices.refresh:
+        index: [test_1]
+
+---
+teardown:
+  - do:
+      indices.delete:
+        index: test_1
+
+# related issue: https://github.com/opensearch-project/OpenSearch/issues/5035
+---
+"Test search_as_you_type data type supports multi-fields":
+  - skip:
+      version: " - 2.17.99"
+      reason: "the bug was fixed since 2.18.0"
+
+  - do:
+      indices.get_mapping: {
+        index: test_1
+      }
+
+  - match: {test_1.mappings.properties.text.type:     search_as_you_type}
+  - match: {test_1.mappings.properties.text.fields.subField.type:     keyword}
+
+  - do:
+      search:
+        index: test_1
+        body:
+          query:
+            multi_match:
+              query: "test search"
+              type: "bool_prefix"
+
+  - match: {hits.total.value: 1}
+
+  - do:
+      search:
+        index: test_1
+        body:
+          query:
+            multi_match:
+              query: "test search"
+              type: "bool_prefix"
+              fields: ["text.subField"]
+
+  - match: {hits.total.value: 1}
+
+  - do:
+      search:
+        index: test_1
+        body:
+          query:
+            term:
+              text.subField: "test search as you type"
+
+  - match: {hits.total.value: 1}