[Backport] [2.x] Fix: Make ChildrenAggregate as a SingleBucketAggrega…

CHANGELOG.md

@@ -22,6 +22,7 @@ Inspired from [Keep a Changelog](https://keepachangelog.com/en/1.0.0/)
 ### Removed
 
 ### Fixed
+- Make ChildrenAggregate as a SingleBucketAggregate ([#306](https://github.com/opensearch-project/opensearch-java/pull/306))
 
 ### Security
 

java-client/src/main/java/org/opensearch/client/opensearch/_types/aggregations/ChildrenAggregate.java

@@ -47,7 +47,7 @@
 // typedef: _types.aggregations.ChildrenAggregate
 
 @JsonpDeserializable
-public class ChildrenAggregate extends MultiBucketAggregateBase<ChildrenAggregateBucket> implements AggregateVariant {
+public class ChildrenAggregate extends SingleBucketAggregateBase implements AggregateVariant {
 	// ---------------------------------------------------------------------------------------------
 
 	private ChildrenAggregate(Builder builder) {
@@ -73,7 +73,7 @@ public Aggregate.Kind _aggregateKind() {
 	 * Builder for {@link ChildrenAggregate}.
 	 */
 
-	public static class Builder extends MultiBucketAggregateBase.AbstractBuilder<ChildrenAggregateBucket, Builder>
+	public static class Builder extends SingleBucketAggregateBase.AbstractBuilder<Builder>
 			implements
 				ObjectBuilder<ChildrenAggregate> {
 		@Override
@@ -89,7 +89,6 @@ protected Builder self() {
 		 */
 		public ChildrenAggregate build() {
 			_checkSingleUse();
-			super.tBucketSerializer(null);
 
 			return new ChildrenAggregate(this);
 		}
@@ -104,8 +103,7 @@ public ChildrenAggregate build() {
 			.lazy(Builder::new, ChildrenAggregate::setupChildrenAggregateDeserializer);
 
 	protected static void setupChildrenAggregateDeserializer(ObjectDeserializer<ChildrenAggregate.Builder> op) {
-		MultiBucketAggregateBase.setupMultiBucketAggregateBaseDeserializer(op, ChildrenAggregateBucket._DESERIALIZER);
-
+		SingleBucketAggregateBase.setupSingleBucketAggregateBaseDeserializer(op);
 	}
 
 }

java-client/src/test/java/org/opensearch/client/opensearch/integTest/RequestTest.java

@@ -374,6 +374,27 @@ public void testSubAggregation() throws IOException {
 
     }
 
+    @Test
+    public void testChildAggregation() throws IOException {
+
+        String index = "test_child";
+        String question = "question";
+        String answer = "answer";
+
+        highLevelClient().indices().create(c -> c.index(index).mappings(m -> m.properties("join", p -> p
+                .join(j -> j.relations(Collections.singletonMap(question, Collections.singletonList(answer)))))));
+        highLevelClient().index(i -> i.index(index).id("1").document(new Question("exists")).refresh(Refresh.True));
+        highLevelClient().index(i -> i.index(index).id("2").routing("1").document(new Answer("true", "1")).refresh(Refresh.True));
+        highLevelClient().index(i -> i.index(index).id("3").routing("1").document(new Answer("false", "1")).refresh(Refresh.True));
+
+        SearchRequest searchRequest = SearchRequest.of(r -> r.index(index).size(0)
+                .aggregations(answer, a -> a.children(c -> c.type(answer))));
+
+        SearchResponse<Void> searchResponse = highLevelClient().search(searchRequest, Void.class);
+
+        assertEquals(2, searchResponse.aggregations().get(answer).children().docCount());
+    }
+
     @Test
     public void testGetMapping() throws Exception {
         // See also VariantsTest.testNestedTaggedUnionWithDefaultTag()
@@ -469,4 +490,48 @@ public void setPrice(double price) {
             this.price = price;
         }
     }
+
+    public static class Join {
+        public String name;
+        public String parent;
+
+        Join() {}
+
+        Join(String name) {
+            this.name = name;
+        }
+
+        Join(String name, String parent) {
+            this.name = name;
+            this.parent = parent;
+        }
+    }
+
+    public static class Question {
+        public String title;
+        public Join join;
+
+        Question() {}
+
+        Question(String title) {
+            this.title = title;
+            this.join = new Join("question");
+        }
+    }
+
+    public static class Answer {
+        public String body;
+        public Join join;
+
+        Answer() {}
+
+        Answer(String body) {
+            this.body = body;
+        }
+
+        Answer(String body, String parent) {
+            this.body = body;
+            this.join = new Join("answer", parent);
+        }
+    }
 }