#344 - Add support for support distinct derived query methods.

We now support distinct derived queries that are useful with projections to retrieve distinct results.

interface UserRepository extends Repository<User, Long> {

  Mono<UserProjection> findDistinctByFirstName(String firstName);

}

interface UserProjection {

  String getFirstName();

}

Original pull request: #346.

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

@@ -41,6 +41,7 @@
  *
  * @author Mark Paluch
  * @author Roman Chigvintsev
+ * @author Mingyuan Wu
  */
 class DefaultStatementMapper implements StatementMapper {
 
@@ -84,8 +85,11 @@ private PreparedOperation<Select> getMappedObject(SelectSpec selectSpec,
 			@Nullable RelationalPersistentEntity<?> entity) {
 
 		Table table = selectSpec.getTable();
-		SelectBuilder.SelectFromAndJoin selectBuilder = StatementBuilder.select(getSelectList(selectSpec, entity))
-				.from(table);
+		SelectBuilder.SelectAndFrom selectAndFrom = StatementBuilder.select(getSelectList(selectSpec, entity));
+		if(selectSpec.isDistinct()){
+			selectAndFrom = selectAndFrom.distinct();
+		}
+		SelectBuilder.SelectFromAndJoin selectBuilder = selectAndFrom.from(table);
 
 		BindMarkers bindMarkers = this.dialect.getBindMarkersFactory().create();
 		Bindings bindings = Bindings.empty();

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

@@ -202,16 +202,18 @@ class SelectSpec {
 		private final Sort sort;
 		private final long offset;
 		private final int limit;
+		private boolean distinct = false;
 
 		protected SelectSpec(Table table, List<String> projectedFields, List<Expression> selectList,
-				@Nullable CriteriaDefinition criteria, Sort sort, int limit, long offset) {
+				@Nullable CriteriaDefinition criteria, Sort sort, int limit, long offset, boolean distinct) {
 			this.table = table;
 			this.projectedFields = projectedFields;
 			this.selectList = selectList;
 			this.criteria = criteria;
 			this.sort = sort;
 			this.offset = offset;
 			this.limit = limit;
+			this.distinct = distinct;
 		}
 
 		/**
@@ -236,7 +238,7 @@ public static SelectSpec create(SqlIdentifier table) {
 			List<String> projectedFields = Collections.emptyList();
 			List<Expression> selectList = Collections.emptyList();
 			return new SelectSpec(Table.create(table), projectedFields, selectList, Criteria.empty(), Sort.unsorted(), -1,
-					-1);
+					-1, false);
 		}
 
 		public SelectSpec doWithTable(BiFunction<Table, SelectSpec, SelectSpec> function) {
@@ -277,7 +279,7 @@ public SelectSpec withProjection(Expression... expressions) {
 			List<Expression> selectList = new ArrayList<>(this.selectList);
 			selectList.addAll(Arrays.asList(expressions));
 
-			return new SelectSpec(this.table, projectedFields, selectList, this.criteria, this.sort, this.limit, this.offset);
+			return new SelectSpec(this.table, projectedFields, selectList, this.criteria, this.sort, this.limit, this.offset, this.distinct);
 		}
 
 		/**
@@ -293,7 +295,7 @@ public SelectSpec withProjection(Collection<Expression> projectedFields) {
 			selectList.addAll(projectedFields);
 
 			return new SelectSpec(this.table, this.projectedFields, selectList, this.criteria, this.sort, this.limit,
-					this.offset);
+					this.offset, this.distinct);
 		}
 
 		/**
@@ -304,7 +306,7 @@ public SelectSpec withProjection(Collection<Expression> projectedFields) {
 		 */
 		public SelectSpec withCriteria(CriteriaDefinition criteria) {
 			return new SelectSpec(this.table, this.projectedFields, this.selectList, criteria, this.sort, this.limit,
-					this.offset);
+					this.offset, this.distinct);
 		}
 
 		/**
@@ -317,11 +319,11 @@ public SelectSpec withSort(Sort sort) {
 
 			if (sort.isSorted()) {
 				return new SelectSpec(this.table, this.projectedFields, this.selectList, this.criteria, sort, this.limit,
-						this.offset);
+						this.offset, this.distinct);
 			}
 
 			return new SelectSpec(this.table, this.projectedFields, this.selectList, this.criteria, this.sort, this.limit,
-					this.offset);
+					this.offset, this.distinct);
 		}
 
 		/**
@@ -337,33 +339,44 @@ public SelectSpec withPage(Pageable page) {
 				Sort sort = page.getSort();
 
 				return new SelectSpec(this.table, this.projectedFields, this.selectList, this.criteria,
-						sort.isSorted() ? sort : this.sort, page.getPageSize(), page.getOffset());
+						sort.isSorted() ? sort : this.sort, page.getPageSize(), page.getOffset(), this.distinct);
 			}
 
 			return new SelectSpec(this.table, this.projectedFields, this.selectList, this.criteria, this.sort, this.limit,
-					this.offset);
+					this.offset, this.distinct);
 		}
 
 		/**
 		 * Associate a result offset with the select and create a new {@link SelectSpec}.
 		 *
-		 * @param page
+		 * @param offset
 		 * @return the {@link SelectSpec}.
 		 */
 		public SelectSpec offset(long offset) {
 			return new SelectSpec(this.table, this.projectedFields, this.selectList, this.criteria, this.sort, this.limit,
-					offset);
+					offset, this.distinct);
 		}
 
 		/**
 		 * Associate a result limit with the select and create a new {@link SelectSpec}.
 		 *
-		 * @param page
+		 * @param limit
 		 * @return the {@link SelectSpec}.
 		 */
 		public SelectSpec limit(int limit) {
 			return new SelectSpec(this.table, this.projectedFields, this.selectList, this.criteria, this.sort, limit,
-					this.offset);
+					this.offset, this.distinct);
+		}
+
+		/**
+		 * Associate a result statement distinct with the select and create a new {@link SelectSpec}.
+		 *
+		 * @param distinct
+		 * @return the {@link SelectSpec}.
+		 */
+		public SelectSpec distinct(boolean distinct) {
+			return new SelectSpec(this.table, this.projectedFields, this.selectList, this.criteria, this.sort, limit,
+					this.offset, distinct);
 		}
 
 		public Table getTable() {
@@ -399,6 +412,10 @@ public long getOffset() {
 		public int getLimit() {
 			return this.limit;
 		}
+
+		public boolean isDistinct() {
+			return this.distinct;
+		}
 	}
 
 	/**

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

@@ -123,6 +123,10 @@ private PreparedOperation<?> select(Criteria criteria, Sort sort, StatementMappe
 			selectSpec = selectSpec.withSort(getSort(sort));
 		}
 
+		if(tree.isDistinct()){
+			selectSpec = selectSpec.distinct(true);
+		}
+
 		return statementMapper.getMappedObject(selectSpec);
 	}
 

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

@@ -63,6 +63,7 @@ public class PartTreeR2dbcQueryUnitTests {
 	private static final String TABLE = "users";
 	private static final String ALL_FIELDS = TABLE + ".id, " + TABLE + ".first_name, " + TABLE + ".last_name, " + TABLE
 			+ ".date_of_birth, " + TABLE + ".age, " + TABLE + ".active";
+	private static final String DISTINCT = "DISTINCT";
 
 	@Mock ConnectionFactory connectionFactory;
 	@Mock R2dbcConverter r2dbcConverter;
@@ -605,6 +606,19 @@ public void createsQueryToDeleteByFirstName() throws Exception {
 		assertThat(bindableQuery.get()).isEqualTo("DELETE FROM " + TABLE + " WHERE " + TABLE + ".first_name = $1");
 	}
 
+	@Test // gh-344
+	public void createsQueryToFindAllEntitiesByStringAttributeWithDistinct() throws Exception {
+
+		R2dbcQueryMethod queryMethod = getQueryMethod("findDistinctByFirstName", String.class);
+		PartTreeR2dbcQuery r2dbcQuery = new PartTreeR2dbcQuery(queryMethod, databaseClient, r2dbcConverter,
+				dataAccessStrategy);
+		BindableQuery bindableQuery = r2dbcQuery.createQuery(getAccessor(queryMethod, new Object[] { "John" }));
+
+		assertThat(bindableQuery.get())
+				.isEqualTo("SELECT " + DISTINCT + " " + ALL_FIELDS + " FROM " + TABLE + " WHERE " + TABLE + ".first_name = $1");
+	}
+
+
 	private R2dbcQueryMethod getQueryMethod(String methodName, Class<?>... parameterTypes) throws Exception {
 		Method method = UserRepository.class.getMethod(methodName, parameterTypes);
 		return new R2dbcQueryMethod(method, new DefaultRepositoryMetadata(UserRepository.class),
@@ -682,6 +696,8 @@ interface UserRepository extends Repository<User, Long> {
 		Flux<User> findTop3ByFirstName(String firstName);
 
 		Mono<User> findFirstByFirstName(String firstName);
+
+		Mono<User> findDistinctByFirstName(String firstName);
 
 		Mono<Integer> deleteByFirstName(String firstName);
 	}