#64 Remove N+1 select by using the OneToManyResultSetExtractor of Spring Data Core JDBC Extensions

pom.xml

@@ -18,6 +18,7 @@
 		<!-- Spring -->
 		<spring-framework.version>4.1.6.RELEASE</spring-framework.version>
 		<spring-data-jpa.version>1.8.0.RELEASE</spring-data-jpa.version>
+		<spring-data-jdbc.version>1.1.0.RELEASE</spring-data-jdbc.version>
 
 
 		<!-- Java EE / Java SE dependencies -->
@@ -127,6 +128,19 @@
 			</exclusions>
 		</dependency>
 
+		<dependency>
+			<groupId>org.springframework.data</groupId>
+			<artifactId>spring-data-jdbc-core</artifactId>
+			<version>${spring-data-jdbc.version}</version>
+			<exclusions>
+				<exclusion>
+					<groupId>org.springframework</groupId>
+					<artifactId>*</artifactId>
+				</exclusion>
+				<!--  because Spring Data usually comes with a slightly older version of Spring -->
+			</exclusions>
+		</dependency>
+
 		<dependency>
 			<groupId>org.springframework</groupId>
 			<artifactId>spring-jdbc</artifactId>

src/main/java/org/springframework/samples/petclinic/repository/jdbc/JdbcOwnerRepositoryImpl.java

@@ -116,18 +116,12 @@ public class JdbcOwnerRepositoryImpl implements OwnerRepository {
         Map<String, Object> params = new HashMap<String, Object>();
         params.put("id", owner.getId().intValue());
         final List<JdbcPet> pets = this.namedParameterJdbcTemplate.query(
-                "SELECT id, name, birth_date, type_id, owner_id FROM pets WHERE owner_id=:id",
+                "SELECT pets.id, name, birth_date, type_id, owner_id, visits.id, visit_date, description, pet_id FROM pets LEFT OUTER JOIN visits ON  pets.id = pet_id WHERE owner_id=:id",
                 params,
-                new JdbcPetRowMapper()
+                new JdbcPetVisitExtractor()
         );
         for (JdbcPet pet : pets) {
             owner.addPet(pet);
-            // Pet types have not been loaded at this stage. They are loaded separately
-            pet.setType(EntityUtils.getById(getPetTypes(), PetType.class, pet.getTypeId()));
-            List<Visit> visits = this.visitRepository.findByPetId(pet.getId());
-            for (Visit visit : visits) {
-                pet.addVisit(visit);
-            }
         }
     }
 

src/main/java/org/springframework/samples/petclinic/repository/jdbc/JdbcPetRowMapper.java

@@ -31,7 +31,7 @@ class JdbcPetRowMapper extends BeanPropertyRowMapper<JdbcPet> {
     @Override
     public JdbcPet mapRow(ResultSet rs, int rownum) throws SQLException {
         JdbcPet pet = new JdbcPet();
-        pet.setId(rs.getInt("id"));
+        pet.setId(rs.getInt("pets.id"));
         pet.setName(rs.getString("name"));
         Date birthDate = rs.getDate("birth_date");
         pet.setBirthDate(new DateTime(birthDate));

src/main/java/org/springframework/samples/petclinic/repository/jdbc/JdbcPetVisitExtractor.java

+/*
+ * Copyright 2002-2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.samples.petclinic.repository.jdbc;
+
+import org.springframework.data.jdbc.core.OneToManyResultSetExtractor;
+import org.springframework.samples.petclinic.model.Visit;
+
+import java.sql.ResultSet;
+import java.sql.SQLException;
+
+public class JdbcPetVisitExtractor extends
+        OneToManyResultSetExtractor<JdbcPet, Visit, Integer> {
+
+    public JdbcPetVisitExtractor() {
+        super(new JdbcPetRowMapper(), new JdbcVisitRowMapper());
+    }
+
+    @Override
+    protected Integer mapPrimaryKey(ResultSet rs) throws SQLException {
+        return rs.getInt("pets.id");
+    }
+
+    @Override
+    protected Integer mapForeignKey(ResultSet rs) throws SQLException {
+        if (rs.getObject("visits.pet_id") == null) {
+            return null;
+        }
+        else {
+            return rs.getInt("visits.pet_id");
+        }
+    }
+
+    @Override
+    protected void addChild(JdbcPet root, Visit child) {
+        root.addVisit(child);
+    }
+}
\ No newline at end of file

src/main/java/org/springframework/samples/petclinic/repository/jdbc/JdbcVisitRepositoryImpl.java

@@ -15,17 +15,8 @@
  */
 package org.springframework.samples.petclinic.repository.jdbc;
 
-import java.sql.ResultSet;
-import java.sql.SQLException;
-import java.util.Date;
-import java.util.List;
-
-import javax.sql.DataSource;
-
-import org.joda.time.DateTime;
 import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.dao.DataAccessException;
-import org.springframework.jdbc.core.BeanPropertyRowMapper;
 import org.springframework.jdbc.core.JdbcTemplate;
 import org.springframework.jdbc.core.namedparam.MapSqlParameterSource;
 import org.springframework.jdbc.core.simple.SimpleJdbcInsert;
@@ -33,6 +24,9 @@ import org.springframework.samples.petclinic.model.Visit;
 import org.springframework.samples.petclinic.repository.VisitRepository;
 import org.springframework.stereotype.Repository;
 
+import javax.sql.DataSource;
+import java.util.List;
+
 /**
  * A simple JDBC-based implementation of the {@link VisitRepository} interface.
  *
@@ -90,21 +84,9 @@ public class JdbcVisitRepositoryImpl implements VisitRepository {
 
     @Override
     public List<Visit> findByPetId(Integer petId) {
-        final List<Visit> visits = this.jdbcTemplate.query(
-                "SELECT id, visit_date, description FROM visits WHERE pet_id=?",
-                new BeanPropertyRowMapper<Visit>() {
-                    @Override
-                    public Visit mapRow(ResultSet rs, int row) throws SQLException {
-                        Visit visit = new Visit();
-                        visit.setId(rs.getInt("id"));
-                        Date visitDate = rs.getDate("visit_date");
-                        visit.setDate(new DateTime(visitDate));
-                        visit.setDescription(rs.getString("description"));
-                        return visit;
-                    }
-                },
-                petId);
-        return visits;
+        return this.jdbcTemplate.query(
+                "SELECT id as visit_id, visit_date, description FROM visits WHERE pet_id=?",
+                new JdbcVisitRowMapper(), petId);
     }
 
 }

src/main/java/org/springframework/samples/petclinic/repository/jdbc/JdbcVisitRowMapper.java

+/*
+ * Copyright 2002-2015 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.springframework.samples.petclinic.repository.jdbc;
+
+
+import org.joda.time.DateTime;
+import org.springframework.jdbc.core.RowMapper;
+import org.springframework.samples.petclinic.model.Visit;
+
+import java.sql.ResultSet;
+import java.sql.SQLException;
+import java.util.Date;
+
+class JdbcVisitRowMapper implements RowMapper<Visit> {
+
+    @Override
+    public Visit mapRow(ResultSet rs, int row) throws SQLException {
+        Visit visit = new Visit();
+        visit.setId(rs.getInt("visits.id"));
+        Date visitDate = rs.getDate("visit_date");
+        visit.setDate(new DateTime(visitDate));
+        visit.setDescription(rs.getString("description"));
+        return visit;
+    }
+}