GH-2417 - Improve generated existsById queries.

Closes #2417

Author: meistermeier

src/main/java/org/springframework/data/neo4j/core/Neo4jOperations.java

@@ -172,6 +172,16 @@ public interface Neo4jOperations {
 	 */
 	<T> List<T> findAllById(Iterable<?> ids, Class<T> domainType);
 
+	/**
+	 * Check if an entity for a given id exists in the database.
+	 *
+	 * @param id the id of the entity to check. Must not be {@code null}.
+	 * @param domainType the type of the entity. Must not be {@code null}.
+	 * @param <T> the type of the entity.
+	 * @return If entity exists in the database, true, otherwise false.
+	 */
+	<T> boolean existsById(Object id, Class<T> domainType);
+
 	/**
 	 * Saves an instance of an entity, including all the related entities of the entity.
 	 *

src/main/java/org/springframework/data/neo4j/core/Neo4jTemplate.java

@@ -288,6 +288,20 @@ <T, R> List<R> doFind(@Nullable String cypherQuery, @Nullable Map<String, Object
 				.collect(Collectors.toList());
 	}
 
+	@Override
+	public <T> boolean existsById(Object id, Class<T> domainType) {
+
+		Neo4jPersistentEntity<?> entityMetaData = neo4jMappingContext.getRequiredPersistentEntity(domainType);
+
+		QueryFragmentsAndParameters fragmentsAndParameters = QueryFragmentsAndParameters
+				.forExistsById(entityMetaData, convertIdValues(entityMetaData.getRequiredIdProperty(), id));
+
+		Statement statement = fragmentsAndParameters.getQueryFragments().toStatement();
+		Map<String, Object> parameters = fragmentsAndParameters.getParameters();
+
+		return count(statement, parameters) > 0;
+	}
+
 	@Override
 	public <T> Optional<T> findById(Object id, Class<T> domainType) {
 		Neo4jPersistentEntity<?> entityMetaData = neo4jMappingContext.getRequiredPersistentEntity(domainType);

src/main/java/org/springframework/data/neo4j/core/ReactiveNeo4jOperations.java

@@ -171,6 +171,16 @@ public interface ReactiveNeo4jOperations {
 	 */
 	<T> Flux<T> findAllById(Iterable<?> ids, Class<T> domainType);
 
+	/**
+	 * Check if an entity for a given id exists in the database.
+	 *
+	 * @param id the id of the entity to check. Must not be {@code null}.
+	 * @param domainType the type of the entity. Must not be {@code null}.
+	 * @param <T> the type of the entity.
+	 * @return If entity exists in the database, true, otherwise false.
+	 */
+	<T> Mono<Boolean> existsById(Object id, Class<T> domainType);
+
 	/**
 	 * Saves an instance of an entity, including all the related entities of the entity.
 	 *

src/main/java/org/springframework/data/neo4j/core/ReactiveNeo4jTemplate.java

@@ -263,6 +263,20 @@ <T, R> Flux<R> doFind(@Nullable String cypherQuery, @Nullable Map<String, Object
 				.mapNotNull(converter::convert);
 	}
 
+	@Override
+	public <T> Mono<Boolean> existsById(Object id, Class<T> domainType) {
+
+		Neo4jPersistentEntity<?> entityMetaData = neo4jMappingContext.getRequiredPersistentEntity(domainType);
+
+		QueryFragmentsAndParameters fragmentsAndParameters = QueryFragmentsAndParameters
+				.forExistsById(entityMetaData, convertIdValues(entityMetaData.getRequiredIdProperty(), id));
+
+		Statement statement = fragmentsAndParameters.getQueryFragments().toStatement();
+		Map<String, Object> parameters = fragmentsAndParameters.getParameters();
+
+		return count(statement, parameters).map(r -> r > 0);
+	}
+
 	@Override
 	public <T> Mono<T> findById(Object id, Class<T> domainType) {
 

src/main/java/org/springframework/data/neo4j/core/mapping/CypherGenerator.java

@@ -455,6 +455,11 @@ public Statement prepareDeleteOf(
 				.build();
 	}
 
+	public Collection<Expression> createReturnStatementForExists(Neo4jPersistentEntity<?> nodeDescription) {
+
+		return Collections.singleton(Functions.count(Constants.NAME_OF_TYPED_ROOT_NODE.apply(nodeDescription)));
+	}
+
 	public Collection<Expression> createReturnStatementForMatch(Neo4jPersistentEntity<?> nodeDescription) {
 		return createReturnStatementForMatch(nodeDescription, (pp -> true));
 	}

src/main/java/org/springframework/data/neo4j/repository/query/CypherdslConditionExecutorImpl.java

@@ -131,6 +131,8 @@ public long count(Condition condition) {
 
 	@Override
 	public boolean exists(Condition condition) {
-		return findAll(condition).iterator().hasNext();
+		Statement statement = CypherGenerator.INSTANCE.prepareMatchOf(this.metaData, condition)
+				.returning(Functions.count(asterisk())).build();
+		return this.neo4jOperations.count(statement, statement.getParameters()) > 0;
 	}
 }

src/main/java/org/springframework/data/neo4j/repository/query/QueryFragmentsAndParameters.java

@@ -121,6 +121,17 @@ public static QueryFragmentsAndParameters forFindAll(Neo4jPersistentEntity<?> en
 		return new QueryFragmentsAndParameters(entityMetaData, queryFragments, Collections.emptyMap());
 	}
 
+	public static QueryFragmentsAndParameters forExistsById(Neo4jPersistentEntity<?> entityMetaData, Object idValues) {
+		Map<String, Object> parameters = Collections.singletonMap(Constants.NAME_OF_ID, idValues);
+
+		Condition condition = entityMetaData.getIdExpression().isEqualTo(parameter(Constants.NAME_OF_ID));
+		QueryFragments queryFragments = new QueryFragments();
+		queryFragments.addMatchOn(cypherGenerator.createRootNode(entityMetaData));
+		queryFragments.setCondition(condition);
+		queryFragments.setReturnExpressions(cypherGenerator.createReturnStatementForExists(entityMetaData));
+		return new QueryFragmentsAndParameters(entityMetaData, queryFragments, parameters);
+	}
+
 	/*
 	 * Following methods are used by the Simple(Reactive)QueryByExampleExecutor
 	 */

src/main/java/org/springframework/data/neo4j/repository/query/SimpleQueryByExampleExecutor.java

@@ -101,7 +101,12 @@ public <S extends T> long count(Example<S> example) {
 
 	@Override
 	public <S extends T> boolean exists(Example<S> example) {
-		return findAll(example).iterator().hasNext();
+
+		Predicate predicate = Predicate.create(mappingContext, example);
+		Statement statement = predicate.useWithReadingFragment(cypherGenerator::prepareMatchOf)
+				.returning(Functions.count(asterisk())).build();
+
+		return this.neo4jOperations.count(statement, predicate.getParameters()) > 0;
 	}
 
 	@Override

src/main/java/org/springframework/data/neo4j/repository/support/SimpleNeo4jRepository.java

@@ -108,7 +108,8 @@ public long count() {
 
 	@Override
 	public boolean existsById(ID id) {
-		return findById(id).isPresent();
+
+		return this.neo4jOperations.existsById(id, this.entityInformation.getJavaType());
 	}
 
 	@Override

src/main/java/org/springframework/data/neo4j/repository/support/SimpleReactiveNeo4jRepository.java

@@ -107,7 +107,7 @@ public Mono<Long> count() {
 
 	@Override
 	public Mono<Boolean> existsById(ID id) {
-		return findById(id).hasElement();
+		return this.neo4jOperations.existsById(id, this.entityInformation.getJavaType());
 	}
 
 	@Override