Fix exists query for imperative repository implementation.

Original Pull Request #2236
Closes #2162

(cherry picked from commit 373be49)
(cherry picked from commit be70a39)

Author: sothawo

src/main/java/org/springframework/data/elasticsearch/repository/query/ElasticsearchPartQuery.java

@@ -128,6 +128,9 @@ public Object execute(Object[] parameters) {
 
 		} else if (tree.isCountProjection()) {
 			result = elasticsearchOperations.count(query, clazz, index);
+		} else if (tree.isExistsProjection()) {
+			long count = elasticsearchOperations.count(query, clazz, index);
+			result = count > 0;
 		} else {
 			result = elasticsearchOperations.searchOne(query, clazz, index);
 		}

src/test/java/org/springframework/data/elasticsearch/repository/query/keywords/QueryKeywordsTests.java

@@ -321,6 +321,17 @@ void shouldFindByNonEmptyProperty() {
 		assertThat(searchHits.getTotalHits()).isEqualTo(5);
 	}
 
+	@Test // #2162
+	@DisplayName("should run exists query")
+	void shouldRunExistsQuery() {
+
+		Boolean existsCaneSugar = repository.existsByText("Cane sugar");
+		Boolean existsSand = repository.existsByText("Sand");
+
+		assertThat(existsCaneSugar).isTrue();
+		assertThat(existsSand).isFalse();
+	}
+
 	@SuppressWarnings("unused")
 	@Document(indexName = "test-index-product-query-keywords")
 	static class Product {
@@ -459,6 +470,8 @@ interface ProductRepository extends ElasticsearchRepository<Product, String> {
 		SearchHits<Product> findByNameEmpty();
 
 		SearchHits<Product> findByNameNotEmpty();
+
+		Boolean existsByText(String text);
 	}
 
 }

src/test/java/org/springframework/data/elasticsearch/repository/query/keywords/ReactiveQueryKeywordsIntegrationTests.java

@@ -19,8 +19,11 @@
 import static org.springframework.data.elasticsearch.annotations.FieldType.*;
 
 import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
 import reactor.test.StepVerifier;
 
+import java.lang.Boolean;
+
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.DisplayName;
 import org.junit.jupiter.api.Order;
@@ -139,6 +142,21 @@ void shouldFindByNotEmptyProperty() {
 				}).verifyComplete();
 	}
 
+	@Test // #2162
+	@DisplayName("should run exists query")
+	void shouldRunExistsQuery() {
+
+		loadEntities();
+		repository.existsByMessage("message") //
+				.as(StepVerifier::create) //
+				.expectNext(true) //
+				.verifyComplete();
+		repository.existsByMessage("without") //
+				.as(StepVerifier::create) //
+				.expectNext(false) //
+				.verifyComplete();
+	}
+
 	@SuppressWarnings("SpringDataMethodInconsistencyInspection")
 	interface SampleRepository extends ReactiveElasticsearchRepository<SampleEntity, String> {
 		Flux<SearchHit<SampleEntity>> findByMessageExists();
@@ -150,6 +168,8 @@ interface SampleRepository extends ReactiveElasticsearchRepository<SampleEntity,
 		Flux<SearchHit<SampleEntity>> findByMessageIsNotEmpty();
 
 		Flux<SearchHit<SampleEntity>> findByMessageIsEmpty();
+
+		Mono<Boolean> existsByMessage(String message);
 	}
 
 	private void loadEntities() {