Fix COUNT/EXISTS projections for entities without an identifier.

We now issue a COUNT(1) respective SELECT 1 for COUNT queries and EXISTS queries for entities that do not specify an identifier.

Previously these query projections could fail because of empty select lists.

Closes #773

src/main/java/org/springframework/data/r2dbc/core/R2dbcEntityTemplate.java

@@ -327,7 +327,7 @@ public class R2dbcEntityTemplate implements R2dbcEntityOperations, BeanFactoryAw
 
 					Expression countExpression = entity.hasIdProperty()
 							? table.column(entity.getRequiredIdProperty().getColumnName())
-							: Expressions.asterisk();
+							: Expressions.just("1");
 					return spec.withProjection(Functions.count(countExpression));
 				});
 
@@ -362,13 +362,14 @@ public class R2dbcEntityTemplate implements R2dbcEntityOperations, BeanFactoryAw
 		RelationalPersistentEntity<?> entity = getRequiredEntity(entityClass);
 		StatementMapper statementMapper = dataAccessStrategy.getStatementMapper().forType(entityClass);
 
-		SqlIdentifier columnName = entity.hasIdProperty() ? entity.getRequiredIdProperty().getColumnName()
+		StatementMapper.SelectSpec selectSpec = statementMapper.createSelect(tableName).limit(1);
-				: SqlIdentifier.unquoted("*");
+		if (entity.hasIdProperty()) {
+			selectSpec = selectSpec //
+					.withProjection(entity.getRequiredIdProperty().getColumnName());
 
-		StatementMapper.SelectSpec selectSpec = statementMapper //
+		} else {
-				.createSelect(tableName) //
+			selectSpec = selectSpec.withProjection(Expressions.just("1"));
-				.withProjection(columnName) //
+		}
-				.limit(1);
 
 		Optional<CriteriaDefinition> criteria = query.getCriteria();
 		if (criteria.isPresent()) {

src/main/java/org/springframework/data/r2dbc/query/QueryMapper.java

@@ -153,7 +153,8 @@ public class QueryMapper {
 	 */
 	public Expression getMappedObject(Expression expression, @Nullable RelationalPersistentEntity<?> entity) {
 
-		if (entity == null || expression instanceof AsteriskFromTable) {
+		if (entity == null || expression instanceof AsteriskFromTable
+				|| expression instanceof Expressions.SimpleExpression) {
 			return expression;
 		}
 

src/main/java/org/springframework/data/r2dbc/repository/query/R2dbcQueryCreator.java

@@ -154,26 +154,22 @@ class R2dbcQueryCreator extends RelationalQueryCreator<PreparedOperation<?>> {
 			for (String projectedProperty : projectedProperties) {
 
 				RelationalPersistentProperty property = entity.getPersistentProperty(projectedProperty);
-				Column column = table.column(property != null ? property.getColumnName() : SqlIdentifier.unquoted(projectedProperty));
+				Column column = table
+						.column(property != null ? property.getColumnName() : SqlIdentifier.unquoted(projectedProperty));
 				expressions.add(column);
 			}
 
-		} else if (tree.isExistsProjection()) {
+		} else if (tree.isExistsProjection() || tree.isCountProjection()) {
-
-			expressions = dataAccessStrategy.getIdentifierColumns(entityToRead).stream()
-				.map(table::column)
-				.collect(Collectors.toList());
-		} else if (tree.isCountProjection()) {
 
 			Expression countExpression = entityMetadata.getTableEntity().hasIdProperty()
 					? table.column(entityMetadata.getTableEntity().getRequiredIdProperty().getColumnName())
-					: Expressions.asterisk();
+					: Expressions.just("1");
 
-			expressions = Collections.singletonList(Functions.count(countExpression));
+			expressions = Collections
+					.singletonList(tree.isCountProjection() ? Functions.count(countExpression) : countExpression);
 		} else {
-			expressions = dataAccessStrategy.getAllColumns(entityToRead).stream()
+			expressions = dataAccessStrategy.getAllColumns(entityToRead).stream().map(table::column)
-				.map(table::column)
+					.collect(Collectors.toList());
-				.collect(Collectors.toList());
 		}
 
 		return expressions.toArray(new Expression[0]);

src/test/java/org/springframework/data/r2dbc/core/R2dbcEntityTemplateUnitTests.java

@@ -90,8 +90,7 @@ public class R2dbcEntityTemplateUnitTests {
 
 		MockRowMetadata metadata = MockRowMetadata.builder()
 				.columnMetadata(MockColumnMetadata.builder().name("name").type(R2dbcType.VARCHAR).build()).build();
-		MockResult result = MockResult.builder()
+		MockResult result = MockResult.builder().row(MockRow.builder().identified(0, Long.class, 1L).build()).build();
-				.row(MockRow.builder().identified(0, Long.class, 1L).build()).build();
 
 		recorder.addStubbing(s -> s.startsWith("SELECT"), result);
 
@@ -109,10 +108,7 @@ public class R2dbcEntityTemplateUnitTests {
 	@Test // gh-469
 	void shouldProjectExistsResult() {
 
-		MockRowMetadata metadata = MockRowMetadata.builder()
+		MockResult result = MockResult.builder().row(MockRow.builder().identified(0, Object.class, null).build()).build();
-				.columnMetadata(MockColumnMetadata.builder().name("name").type(R2dbcType.VARCHAR).build()).build();
-		MockResult result = MockResult.builder()
-				.row(MockRow.builder().identified(0, Object.class, null).build()).build();
 
 		recorder.addStubbing(s -> s.startsWith("SELECT"), result);
 
@@ -124,13 +120,36 @@ public class R2dbcEntityTemplateUnitTests {
 				.verifyComplete();
 	}
 
+	@Test // gh-773
+	void shouldProjectExistsResultWithoutId() {
+
+		MockResult result = MockResult.builder().row(MockRow.builder().identified(0, Object.class, null).build()).build();
+
+		recorder.addStubbing(s -> s.startsWith("SELECT 1"), result);
+
+		entityTemplate.select(WithoutId.class).exists() //
+				.as(StepVerifier::create) //
+				.expectNext(true).verifyComplete();
+	}
+
+	@Test // gh-773
+	void shouldProjectCountResultWithoutId() {
+
+		MockResult result = MockResult.builder().row(MockRow.builder().identified(0, Long.class, 1L).build()).build();
+
+		recorder.addStubbing(s -> s.startsWith("SELECT COUNT(1)"), result);
+
+		entityTemplate.select(WithoutId.class).count() //
+				.as(StepVerifier::create) //
+				.expectNext(1L).verifyComplete();
+	}
+
 	@Test // gh-469
 	void shouldExistsByCriteria() {
 
 		MockRowMetadata metadata = MockRowMetadata.builder()
 				.columnMetadata(MockColumnMetadata.builder().name("name").type(R2dbcType.VARCHAR).build()).build();
-		MockResult result = MockResult.builder()
+		MockResult result = MockResult.builder().row(MockRow.builder().identified(0, Long.class, 1L).build()).build();
-				.row(MockRow.builder().identified(0, Long.class, 1L).build()).build();
 
 		recorder.addStubbing(s -> s.startsWith("SELECT"), result);
 
@@ -480,6 +499,12 @@ public class R2dbcEntityTemplateUnitTests {
 				Parameter.from("before-save"));
 	}
 
+	@Value
+	static class WithoutId {
+
+		String name;
+	}
+
 	@Value
 	@With
 	static class Person {

src/test/java/org/springframework/data/r2dbc/repository/query/PartTreeR2dbcQueryUnitTests.java

@@ -77,8 +77,6 @@ class PartTreeR2dbcQueryUnitTests {
 			".age", ".active" };
 	private static final String[] ALL_FIELDS_ARRAY_PREFIXED = Arrays.stream(ALL_FIELDS_ARRAY).map(f -> TABLE + f)
 			.toArray(String[]::new);
-	private static final String ALL_FIELDS = String.join(", ", ALL_FIELDS_ARRAY_PREFIXED);
-	private static final String DISTINCT = "DISTINCT";
 
 	@Mock ConnectionFactory connectionFactory;
 	@Mock R2dbcConverter r2dbcConverter;
@@ -698,6 +696,32 @@ class PartTreeR2dbcQueryUnitTests {
 				.where(TABLE + ".first_name = $1");
 	}
 
+	@Test // GH-773
+	void createsQueryWithoutIdForCountProjection() throws Exception {
+
+		R2dbcQueryMethod queryMethod = getQueryMethod(WithoutIdRepository.class, "countByFirstName", String.class);
+		PartTreeR2dbcQuery r2dbcQuery = new PartTreeR2dbcQuery(queryMethod, operations, r2dbcConverter, dataAccessStrategy);
+		PreparedOperation<?> query = createQuery(queryMethod, r2dbcQuery, "John");
+
+		PreparedOperationAssert.assertThat(query) //
+				.selects("COUNT(1)") //
+				.from(TABLE) //
+				.where(TABLE + ".first_name = $1");
+	}
+
+	@Test // GH-773
+	void createsQueryWithoutIdForExistsProjection() throws Exception {
+
+		R2dbcQueryMethod queryMethod = getQueryMethod(WithoutIdRepository.class, "existsByFirstName", String.class);
+		PartTreeR2dbcQuery r2dbcQuery = new PartTreeR2dbcQuery(queryMethod, operations, r2dbcConverter, dataAccessStrategy);
+		PreparedOperation<?> query = createQuery(queryMethod, r2dbcQuery, "John");
+
+		PreparedOperationAssert.assertThat(query) //
+				.selects("1") //
+				.from(TABLE) //
+				.where(TABLE + ".first_name = $1 LIMIT 1");
+	}
+
 	private PreparedOperation<?> createQuery(R2dbcQueryMethod queryMethod, PartTreeR2dbcQuery r2dbcQuery,
 			Object... parameters) {
 		return createQuery(r2dbcQuery, getAccessor(queryMethod, parameters));
@@ -709,8 +733,13 @@ class PartTreeR2dbcQueryUnitTests {
 	}
 
 	private R2dbcQueryMethod getQueryMethod(String methodName, Class<?>... parameterTypes) throws Exception {
-		Method method = UserRepository.class.getMethod(methodName, parameterTypes);
+		return getQueryMethod(UserRepository.class, methodName, parameterTypes);
-		return new R2dbcQueryMethod(method, new DefaultRepositoryMetadata(UserRepository.class),
+	}
+
+	private R2dbcQueryMethod getQueryMethod(Class<?> repository, String methodName, Class<?>... parameterTypes)
+			throws Exception {
+		Method method = repository.getMethod(methodName, parameterTypes);
+		return new R2dbcQueryMethod(method, new DefaultRepositoryMetadata(repository),
 				new SpelAwareProxyProjectionFactory(), mappingContext);
 	}
 
@@ -887,6 +916,13 @@ class PartTreeR2dbcQueryUnitTests {
 		Mono<Long> countByFirstName(String firstName);
 	}
 
+	interface WithoutIdRepository extends Repository<WithoutId, Long> {
+
+		Mono<Boolean> existsByFirstName(String firstName);
+
+		Mono<Long> countByFirstName(String firstName);
+	}
+
 	@Table("users")
 	@Data
 	private static class User {
@@ -899,6 +935,13 @@ class PartTreeR2dbcQueryUnitTests {
 		private Boolean active;
 	}
 
+	@Table("users")
+	@Data
+	private static class WithoutId {
+
+		private String firstName;
+	}
+
 	interface UserProjection {
 
 		String getFirstName();