Enable use of search_after with field_collapse

src/main/java/org/springframework/data/elasticsearch/client/elc/ReactiveElasticsearchTemplate.java

@@ -395,7 +395,28 @@ private Flux<SearchDocument> doFindUnbounded(Query query, Class<?> clazz, IndexC
 			Function<PitSearchAfter, Publisher<? extends ResponseBody<EntityAsMap>>> resourceClosure = psa -> {
 
 				baseQuery.setPointInTime(new Query.PointInTime(psa.getPit(), pitKeepAlive));
-				baseQuery.addSort(Sort.by("_shard_doc"));
+
+				// only add _shard_doc if there is not a field_collapse and a sort with the same name
+				boolean addShardDoc = true;
+
+				if (query instanceof NativeQuery nativeQuery && nativeQuery.getFieldCollapse() != null) {
+					var field = nativeQuery.getFieldCollapse().field();
+
+					if (nativeQuery.getSortOptions().stream()
+							.anyMatch(sortOptions -> sortOptions.isField() && sortOptions.field().field().equals(field))) {
+						addShardDoc = false;
+					}
+
+					if (query.getSort() != null
+							&& query.getSort().stream().anyMatch(order -> order.getProperty().equals(field))) {
+						addShardDoc = false;
+					}
+				}
+
+				if (addShardDoc) {
+					baseQuery.addSort(Sort.by("_shard_doc"));
+				}
+
 				SearchRequest firstSearchRequest = requestConverter.searchRequest(baseQuery, routingResolver.getRouting(),
 						clazz, index, false, true);
 

src/main/java/org/springframework/data/elasticsearch/client/elc/RequestConverter.java

@@ -1487,8 +1487,8 @@ private <T> void prepareSearchRequest(Query query, @Nullable String routing, @Nu
 		if (query instanceof NativeQuery nativeQuery) {
 			prepareNativeSearch(nativeQuery, builder);
 		}
-		// query.getSort() must be checked after prepareNativeSearch as this already might hav a sort set that must have
-		// higher priority
+		// query.getSort() must be checked after prepareNativeSearch as this already might have a sort set
+		// that must have higher priority
 		if (query.getSort() != null) {
 			List<SortOptions> sortOptions = getSortOptions(query.getSort(), persistentEntity);
 
@@ -1510,7 +1510,15 @@ private <T> void prepareSearchRequest(Query query, @Nullable String routing, @Nu
 		}
 
 		if (!isEmpty(query.getSearchAfter())) {
-			builder.searchAfter(query.getSearchAfter().stream().map(TypeUtils::toFieldValue).toList());
+			var fieldValues = query.getSearchAfter().stream().map(TypeUtils::toFieldValue).toList();
+
+			// when there is a field collapse on a native query, and we have a search_after, then the search_after
+			// must only have one entry
+			if (query instanceof NativeQuery nativeQuery && nativeQuery.getFieldCollapse() != null) {
+				builder.searchAfter(fieldValues.get(0));
+			} else {
+				builder.searchAfter(fieldValues);
+			}
 		}
 
 		query.getRescorerQueries().forEach(rescorerQuery -> builder.rescore(getRescore(rescorerQuery)));

src/test/java/org/springframework/data/elasticsearch/repository/support/SimpleReactiveElasticsearchRepositoryELCIntegrationTests.java

@@ -15,14 +15,22 @@
  */
 package org.springframework.data.elasticsearch.repository.support;
 
+import co.elastic.clients.elasticsearch.core.search.FieldCollapse;
+import org.junit.jupiter.api.DisplayName;
+import org.junit.jupiter.api.Test;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Import;
+import org.springframework.data.domain.Pageable;
+import org.springframework.data.domain.Sort;
+import org.springframework.data.elasticsearch.client.elc.NativeQuery;
+import org.springframework.data.elasticsearch.client.elc.Queries;
 import org.springframework.data.elasticsearch.junit.jupiter.ReactiveElasticsearchTemplateConfiguration;
 import org.springframework.data.elasticsearch.repositories.custommethod.QueryParameter;
 import org.springframework.data.elasticsearch.repository.config.EnableReactiveElasticsearchRepositories;
 import org.springframework.data.elasticsearch.utils.IndexNameProvider;
 import org.springframework.test.context.ContextConfiguration;
+import reactor.test.StepVerifier;
 
 /**
  * @author Peter-Josef Meisch
@@ -51,4 +59,33 @@ QueryParameter queryParameter() {
 		}
 	}
 
+	/**
+	 * search_after is used by the reactive search operation, it normally always adds _shard_doc as a tiebreaker sort
+	 * parameter. This must not be done when a collapse field is used as sort field, as in that case the collapse field
+	 * must be the only sort field.
+	 */
+	@Test // #2935
+	@DisplayName("should use collapse_field for search_after in pit search")
+	void shouldUseCollapseFieldForSearchAfterI() {
+		var entity = new SampleEntity();
+		entity.setId("42");
+		entity.setMessage("m");
+		entity.setKeyword("kw");
+		repository.save(entity).block();
+
+		var query = NativeQuery.builder()
+				.withQuery(Queries.matchAllQueryAsQuery())
+				.withPageable(Pageable.unpaged())
+				.withFieldCollapse(FieldCollapse.of(fcb -> fcb
+						.field("keyword")))
+				.withSort(Sort.by("keyword"))
+				.build();
+
+		operations.search(query, SampleEntity.class)
+				.as(StepVerifier::create)
+				.expectNextCount(1)
+				.verifyComplete();
+	}
+
+
 }