Rewrite properly recreates the original KNNQueryBuilder with null k

CHANGELOG.md

@@ -4,4 +4,6 @@ All notable changes to this project are documented in this file.
 
 The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/), and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html). See the [CONTRIBUTING guide](./CONTRIBUTING.md#Changelog) for instructions on how to add changelog entries.
 
-## [Unreleased 3.2](https://github.com/opensearch-project/k-NN/compare/main...HEAD)
+## Bug Fixes
+
+* Reverts k back to null in radial search with filter rewrite [#2915](https://github.com/opensearch-project/k-NN/pull/2915)

src/main/java/org/opensearch/knn/index/query/KNNQueryBuilder.java

@@ -740,10 +740,11 @@ protected QueryBuilder doRewrite(QueryRewriteContext queryShardContext) throws I
         if (Objects.nonNull(filter)) {
             rewrittenFilter = filter.rewrite(queryShardContext);
             if (rewrittenFilter != filter) {
+                Integer optionalK = this.k == 0 ? null : this.k;
                 KNNQueryBuilder rewrittenQueryBuilder = KNNQueryBuilder.builder()
                     .fieldName(this.fieldName)
                     .vector(this.vector)
-                    .k(this.k)
+                    .k(optionalK)
                     .maxDistance(this.maxDistance)
                     .minScore(this.minScore)
                     .methodParameters(this.methodParameters)

src/test/java/org/opensearch/knn/index/query/KNNQueryBuilderTests.java

@@ -540,6 +540,7 @@ public void testDoToQuery_whenDoRadiusSearch_whenDistanceThreshold_whenFilter_th
             org.opensearch.knn.common.KNNConstants.METHOD_HNSW,
             ImmutableMap.of()
         );
+        knnQueryBuilder = (KNNQueryBuilder) knnQueryBuilder.doRewrite(mockQueryShardContext);
         KNNMethodContext knnMethodContext = new KNNMethodContext(KNNEngine.LUCENE, SpaceType.L2, methodComponentContext);
         when(mockKNNVectorField.getKnnMappingConfig()).thenReturn(getMappingConfigForMethodMapping(knnMethodContext, 4));
         when(mockQueryShardContext.fieldMapper(anyString())).thenReturn(mockKNNVectorField);