sample codes for article BAEL-3581 (#8942)

persistence-modules/hibernate-jpa/pom.xml

@@ -81,11 +81,17 @@
             <artifactId>jmh-generator-annprocess</artifactId>
             <version>${openjdk-jmh.version}</version>
         </dependency>
+        <dependency>
+            <groupId>org.postgresql</groupId>
+            <artifactId>postgresql</artifactId>
+            <version>${postgresql.version}</version>
+        </dependency>
     </dependencies>
 
     <properties>
         <hibernate.version>5.3.7.Final</hibernate.version>
-        <mysql.version>6.0.6</mysql.version>
+        <mysql.version>8.0.13</mysql.version>
+        <postgresql.version>42.2.11</postgresql.version>
         <mariaDB4j.version>2.2.3</mariaDB4j.version>
         <assertj-core.version>3.8.0</assertj-core.version>
         <openjdk-jmh.version>1.21</openjdk-jmh.version>

persistence-modules/hibernate-jpa/src/main/java/com/baeldung/hibernate/entitymanager/getreference/Game.java

@@ -0,0 +1,38 @@
+package com.baeldung.hibernate.entitymanager.getreference;
+
+import javax.persistence.Entity;
+import javax.persistence.Id;
+
+@Entity
+public class Game {
+
+    @Id
+    private Long id;
+
+    private String name;
+
+    public Game() {
+    }
+
+    public Game(Long id, String name) {
+        this.id = id;
+        this.name = name;
+    }
+
+    public Long getId() {
+        return id;
+    }
+
+    public void setId(Long id) {
+        this.id = id;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+}

persistence-modules/hibernate-jpa/src/main/java/com/baeldung/hibernate/entitymanager/getreference/Player.java

@@ -0,0 +1,50 @@
+package com.baeldung.hibernate.entitymanager.getreference;
+
+import javax.persistence.Entity;
+import javax.persistence.Id;
+import javax.persistence.ManyToOne;
+
+@Entity
+public class Player {
+
+    @Id
+    private Long id;
+
+    private String name;
+
+    @ManyToOne
+    private Game game;
+
+    public Player() {
+    }
+
+    public Player(Long id, String name) {
+        this.id = id;
+        this.name = name;
+    }
+
+    public Long getId() {
+        return id;
+    }
+
+    public void setId(Long id) {
+        this.id = id;
+    }
+
+    public String getName() {
+        return name;
+    }
+
+    public void setName(String name) {
+        this.name = name;
+    }
+
+    public Game getGame() {
+        return game;
+    }
+
+    public void setGame(Game game) {
+        this.game = game;
+    }
+
+}

persistence-modules/hibernate-jpa/src/main/resources/META-INF/create-db.sql

@@ -0,0 +1,3 @@
+create table Game (id bigint not null, name varchar(255), primary key (id));
+create table Player (id bigint not null, name varchar(255), game_id bigint, primary key (id));
+alter table Player add constraint FKohr86afuapoujklti79wo27aa foreign key (game_id) references Game(id);

persistence-modules/hibernate-jpa/src/main/resources/META-INF/data.sql

@@ -0,0 +1,5 @@
+insert into Game (id, name) values (1, 'Game 1');
+insert into Game (id, name) values (2, 'Game 2');
+insert into Player (game_id, name, id) values (null, 'Player 1', 1);
+insert into Player (game_id, name, id) values (null, 'Player 2', 2);
+insert into Player (game_id, name, id) values (null, 'Player 3', 3);

persistence-modules/hibernate-jpa/src/main/resources/META-INF/drop-db.sql

@@ -0,0 +1,2 @@
+drop table if exists Player;
+drop table if exists Game;

persistence-modules/hibernate-jpa/src/main/resources/META-INF/persistence.xml

@@ -5,7 +5,7 @@
              version="2.0">
     <persistence-unit name="com.baeldung.movie_catalog">
         <description>Hibernate EntityManager Demo</description>
-        <class>com.baeldung.hibernate.pojo.Movie</class> 
+        <class>com.baeldung.hibernate.pojo.Movie</class>
         <exclude-unlisted-classes>true</exclude-unlisted-classes>
         <properties>
             <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5Dialect"/>
@@ -16,4 +16,88 @@
             <property name="javax.persistence.jdbc.password" value="root"/>
         </properties>
     </persistence-unit>
+    <!-- Persistence unit for H2 -->
+    <persistence-unit name="com.baeldung.hibernate.entitymanager.game_player_h2">
+        <description>EntityManager getReference persistence unit</description>
+        <class>com.baeldung.hibernate.entitymanager.getreference.Game</class>
+        <class>com.baeldung.hibernate.entitymanager.getreference.Player</class>
+        <exclude-unlisted-classes>true</exclude-unlisted-classes>
+        <properties>
+            <!-- even if we set exclude-unlisted-classes=true, strangely it's not enough to exclude defined hbm files in hibernate.cfg.xml.
+            So, some entities out of the scope unwillingly managed by our persistence unit. In order to prevent this, we disable autodetection
+            of the hbm files completely. When we set this property, hibernate ignores the exclude-unlisted-classes property. -->
+            <property name="hibernate.archive.autodetection" value=""/>
+
+            <property name="hibernate.jpa.compliance.proxy" value="false"/>
+
+            <property name="hibernate.hbm2ddl.auto" value="update"/>
+            <property name="hibernate.show_sql" value="true"/>
+            <property name="hibernate.generate_statistics" value="false"/>
+            <property name="hibernate.dialect" value="org.hibernate.dialect.H2Dialect"/>
+            <property name="javax.persistence.jdbc.driver" value="org.h2.Driver"/>
+            <property name="javax.persistence.jdbc.url" value="jdbc:h2:mem:db1;DB_CLOSE_DELAY=-1"/>
+            <property name="javax.persistence.jdbc.user" value="sa"/>
+            <property name="javax.persistence.jdbc.password" value=""/>
+        </properties>
+    </persistence-unit>
+    <!-- Persistence unit for MySQL -->
+    <persistence-unit name="com.baeldung.hibernate.entitymanager.game_player_mysql">
+        <description>EntityManager getReference persistence unit</description>
+        <class>com.baeldung.hibernate.entitymanager.getreference.Game</class>
+        <class>com.baeldung.hibernate.entitymanager.getreference.Player</class>
+        <exclude-unlisted-classes>true</exclude-unlisted-classes>
+        <properties>
+            <!-- even if we set exclude-unlisted-classes=true, strangely it's not enough to exclude defined hbm files in hibernate.cfg.xml.
+            So, some entities out of the scope unwillingly managed by our persistence unit. In order to prevent this, we disable autodetection
+            of the hbm files completely. When we set this property, hibernate ignores the exclude-unlisted-classes property. -->
+            <property name="hibernate.archive.autodetection" value=""/>
+
+            <property name="hibernate.jpa.compliance.proxy" value="false"/>
+
+            <property name="hibernate.hbm2ddl.auto" value="update"/>
+            <property name="hibernate.show_sql" value="true"/>
+            <property name="hibernate.generate_statistics" value="false"/>
+            <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5Dialect"/>
+            <property name="javax.persistence.jdbc.driver" value="com.mysql.cj.jdbc.Driver"/>
+            <property name="javax.persistence.jdbc.url" value="jdbc:mysql://localhost:3306/baeldung"/>
+            <property name="javax.persistence.jdbc.user" value="root"/>
+            <property name="javax.persistence.jdbc.password" value="password"/>
+
+            <property name="javax.persistence.schema-generation.database.action" value="drop-and-create"/>
+            <property name="javax.persistence.schema-generation.create-script-source" value="/META-INF/create-db.sql"/>
+            <property name="javax.persistence.schema-generation.drop-script-source" value="/META-INF/drop-db.sql"/>
+            <property name="javax.persistence.sql-load-script-source" value="/META-INF/data.sql"/>
+
+        </properties>
+    </persistence-unit>
+    <!-- Persistence unit for PostgreSQL -->
+    <persistence-unit name="com.baeldung.hibernate.entitymanager.game_player_postgresql">
+        <description>EntityManager getReference persistence unit</description>
+        <class>com.baeldung.hibernate.entitymanager.getreference.Game</class>
+        <class>com.baeldung.hibernate.entitymanager.getreference.Player</class>
+        <exclude-unlisted-classes>true</exclude-unlisted-classes>
+        <properties>
+            <!-- even if we set exclude-unlisted-classes=true, strangely it's not enough to exclude defined hbm files in hibernate.cfg.xml.
+            So, some entities out of the scope unwillingly managed by our persistence unit. In order to prevent this, we disable autodetection
+            of the hbm files completely. When we set this property, hibernate ignores the exclude-unlisted-classes property. -->
+            <property name="hibernate.archive.autodetection" value=""/>
+
+            <property name="hibernate.jpa.compliance.proxy" value="false"/>
+
+            <property name="hibernate.hbm2ddl.auto" value="update"/>
+            <property name="hibernate.show_sql" value="true"/>
+            <property name="hibernate.generate_statistics" value="false"/>
+            <property name="hibernate.dialect" value="org.hibernate.dialect.PostgreSQL95Dialect"/>
+            <property name="javax.persistence.jdbc.driver" value="org.postgresql.Driver"/>
+            <property name="javax.persistence.jdbc.url" value="jdbc:postgresql://localhost:5432/postgres"/>
+            <property name="javax.persistence.jdbc.user" value="postgres"/>
+            <property name="javax.persistence.jdbc.password" value=""/>
+
+            <property name="javax.persistence.schema-generation.database.action" value="drop-and-create"/>
+            <property name="javax.persistence.schema-generation.create-script-source" value="/META-INF/create-db.sql"/>
+            <property name="javax.persistence.schema-generation.drop-script-source" value="/META-INF/drop-db.sql"/>
+            <property name="javax.persistence.sql-load-script-source" value="/META-INF/data.sql"/>
+
+        </properties>
+    </persistence-unit>
 </persistence> 

persistence-modules/hibernate-jpa/src/test/java/com/baeldung/hibernate/entitymanager/getreference/GetReferenceH2IntegrationTest.java

@@ -0,0 +1,194 @@
+package com.baeldung.hibernate.entitymanager.getreference;
+
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.Logger;
+import org.junit.jupiter.api.*;
+import org.slf4j.LoggerFactory;
+
+import javax.persistence.EntityManager;
+import javax.persistence.EntityManagerFactory;
+import javax.persistence.Persistence;
+import java.io.ByteArrayOutputStream;
+import java.io.OutputStream;
+import java.io.PrintStream;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+public class GetReferenceH2IntegrationTest {
+
+    private static EntityManagerFactory entityManagerFactory;
+    private static EntityManager entityManager;
+
+    private static final PrintStream SystemOut = System.out;
+    private static OutputStream output;
+
+    @BeforeAll
+    public static void setup() {
+        // close some specific loggers so that we can clearly see Hibernate: SQL queries
+        ((Logger) LoggerFactory.getLogger("org.hibernate.SQL")).setLevel(Level.OFF);
+        ((Logger) LoggerFactory.getLogger("org.hibernate.type.descriptor.sql")).setLevel(Level.OFF);
+        ((Logger) LoggerFactory.getLogger("org.hibernate.stat")).setLevel(Level.OFF);
+
+        entityManagerFactory = Persistence.createEntityManagerFactory("com.baeldung.hibernate.entitymanager.game_player_h2");
+        entityManager = entityManagerFactory.createEntityManager();
+        entityManager.getTransaction().begin();
+
+        entityManager.persist(new Game(1L, "Game 1"));
+        entityManager.persist(new Game(2L, "Game 2"));
+        entityManager.persist(new Player(1L, "Player 1"));
+        entityManager.persist(new Player(2L, "Player 2"));
+        entityManager.persist(new Player(3L, "Player 3"));
+
+        entityManager.getTransaction().commit();
+        entityManager.clear();
+        entityManager.close();
+    }
+
+    private void runInTransaction(Runnable task) {
+        // We create new persistence context for each test method to discard Hibernate first level cache.
+        // So that we can see the behavior of getReference() method in a non-cached environment.
+        entityManager = entityManagerFactory.createEntityManager();
+        entityManager.getTransaction().begin();
+        task.run();
+        entityManager.getTransaction().commit();
+        // In any case, we use clear() and close() to make all the managed entities detached and cleared.
+        // So, we can be sure we test always on a clear persistence context.
+        entityManager.clear();
+        entityManager.close();
+    }
+
+    @BeforeEach
+    public void beforeEach() {
+        // stubbing System.out printStream
+        output = new ByteArrayOutputStream();
+        System.setOut(new PrintStream(output));
+    }
+
+    @AfterEach
+    public void afterEach() {
+        System.setOut(SystemOut);
+        // we print to see original output after each test method
+        System.out.print(output.toString());
+    }
+
+    @AfterAll
+    public static void tearDown() {
+        entityManagerFactory.close();
+    }
+
+    @Test
+    public void whenUsingFindMethodToUpdateGame_thenExecutesSelectForGame() {
+
+        runInTransaction(() -> {
+            Game game1 = entityManager.find(Game.class, 1L);
+            game1.setName("Game Updated 1");
+
+            entityManager.persist(game1);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select game0_.id as id1_0_0_, game0_.name as name2_0_0_ from Game game0_ where game0_.id=?\n");
+        expected.append("Hibernate: update Game set name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingGetReferenceMethodToUpdateGame_thenExecutesSelectForGame() {
+
+        runInTransaction(() -> {
+            Game game1 = entityManager.getReference(Game.class, 1L);
+            game1.setName("Game Updated 2");
+
+            entityManager.persist(game1);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select game0_.id as id1_0_0_, game0_.name as name2_0_0_ from Game game0_ where game0_.id=?\n");
+        expected.append("Hibernate: update Game set name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingFindMethodToDeletePlayer_thenExecutesSelectForPlayer() {
+
+        runInTransaction(() -> {
+            Player player2 = entityManager.find(Player.class, 2L);
+            entityManager.remove(player2);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: delete from Player where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingGetReferenceMethodToDeletePlayer_thenExecutesSelectForPlayer() {
+
+        runInTransaction(() -> {
+            Player player3 = entityManager.getReference(Player.class, 3L);
+            entityManager.remove(player3);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: delete from Player where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingFindMethodToUpdatePlayersGame_thenExecutesSelectForGame() {
+
+        runInTransaction((() -> {
+            Game game1 = entityManager.find(Game.class, 1L);
+
+            Player player1 = entityManager.find(Player.class, 1L);
+            player1.setGame(game1);
+
+            entityManager.persist(player1);
+        }));
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select game0_.id as id1_0_0_, game0_.name as name2_0_0_ from Game game0_ where game0_.id=?\n");
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: update Player set game_id=?, name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingGetReferenceMethodToUpdatePlayersGame_thenDoesNotExecuteSelectForGame() {
+
+        runInTransaction(() -> {
+            Game game2 = entityManager.getReference(Game.class, 2L);
+
+            Player player1 = entityManager.find(Player.class, 1L);
+            player1.setGame(game2);
+
+            entityManager.persist(player1);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: update Player set game_id=?, name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+}

persistence-modules/hibernate-jpa/src/test/java/com/baeldung/hibernate/entitymanager/getreference/GetReferenceMySQLManualTest.java

@@ -0,0 +1,185 @@
+package com.baeldung.hibernate.entitymanager.getreference;
+
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.Logger;
+import org.junit.jupiter.api.*;
+import org.slf4j.LoggerFactory;
+
+import javax.persistence.EntityManager;
+import javax.persistence.EntityManagerFactory;
+import javax.persistence.Persistence;
+import java.io.ByteArrayOutputStream;
+import java.io.OutputStream;
+import java.io.PrintStream;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * We need to have a running MySQL instance and a schema named "baeldung" ready in order to execute this integration test
+ */
+public class GetReferenceMySQLManualTest {
+
+    private static EntityManagerFactory entityManagerFactory;
+    private static EntityManager entityManager;
+
+    private static final PrintStream SystemOut = System.out;
+    private static OutputStream output;
+
+    @BeforeAll
+    public static void setup() {
+        // close some specific loggers so that we can clearly see Hibernate: SQL queries
+        ((Logger) LoggerFactory.getLogger("org.hibernate.SQL")).setLevel(Level.OFF);
+        ((Logger) LoggerFactory.getLogger("org.hibernate.type.descriptor.sql")).setLevel(Level.OFF);
+        ((Logger) LoggerFactory.getLogger("org.hibernate.stat")).setLevel(Level.OFF);
+
+        entityManagerFactory = Persistence.createEntityManagerFactory("com.baeldung.hibernate.entitymanager.game_player_mysql");
+    }
+
+    private void runInTransaction(Runnable task) {
+        // We create new persistence context for each test method to discard Hibernate first level cache.
+        // So that we can see the behavior of getReference() method in a non-cached environment.
+        entityManager = entityManagerFactory.createEntityManager();
+        entityManager.getTransaction().begin();
+        task.run();
+        entityManager.getTransaction().commit();
+        // In any case, we use clear() and close() to make all the managed entities detached and cleared.
+        // So, we can be sure we test always on a clear persistence context.
+        entityManager.clear();
+        entityManager.close();
+    }
+
+    @BeforeEach
+    public void beforeEach() {
+        // stubbing System.out printStream
+        output = new ByteArrayOutputStream();
+        System.setOut(new PrintStream(output));
+    }
+
+    @AfterEach
+    public void afterEach() {
+        System.setOut(SystemOut);
+        // we print to see original output after each test method
+        System.out.print(output.toString());
+    }
+
+    @AfterAll
+    public static void tearDown() {
+        entityManagerFactory.close();
+    }
+
+    @Test
+    public void whenUsingFindMethodToUpdateGame_thenExecutesSelectForGame() {
+
+        runInTransaction(() -> {
+            Game game1 = entityManager.find(Game.class, 1L);
+            game1.setName("Game Updated 1");
+
+            entityManager.persist(game1);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select game0_.id as id1_0_0_, game0_.name as name2_0_0_ from Game game0_ where game0_.id=?\n");
+        expected.append("Hibernate: update Game set name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingGetReferenceMethodToUpdateGame_thenExecutesSelectForGame() {
+
+        runInTransaction(() -> {
+            Game game1 = entityManager.getReference(Game.class, 1L);
+            game1.setName("Game Updated 2");
+
+            entityManager.persist(game1);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select game0_.id as id1_0_0_, game0_.name as name2_0_0_ from Game game0_ where game0_.id=?\n");
+        expected.append("Hibernate: update Game set name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingFindMethodToDeletePlayer_thenExecutesSelectForPlayer() {
+
+        runInTransaction(() -> {
+            Player player2 = entityManager.find(Player.class, 2L);
+            entityManager.remove(player2);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: delete from Player where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingGetReferenceMethodToDeletePlayer_thenExecutesSelectForPlayer() {
+
+        runInTransaction(() -> {
+            Player player3 = entityManager.getReference(Player.class, 3L);
+            entityManager.remove(player3);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: delete from Player where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingFindMethodToUpdatePlayersGame_thenExecutesSelectForGame() {
+
+        runInTransaction((() -> {
+            Game game1 = entityManager.find(Game.class, 1L);
+
+            Player player1 = entityManager.find(Player.class, 1L);
+            player1.setGame(game1);
+
+            entityManager.persist(player1);
+        }));
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select game0_.id as id1_0_0_, game0_.name as name2_0_0_ from Game game0_ where game0_.id=?\n");
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: update Player set game_id=?, name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingGetReferenceMethodToUpdatePlayersGame_thenDoesNotExecuteSelectForGame() {
+
+        runInTransaction(() -> {
+            Game game2 = entityManager.getReference(Game.class, 2L);
+
+            Player player1 = entityManager.find(Player.class, 1L);
+            player1.setGame(game2);
+
+            entityManager.persist(player1);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: update Player set game_id=?, name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+}

persistence-modules/hibernate-jpa/src/test/java/com/baeldung/hibernate/entitymanager/getreference/GetReferencePostgreSQLManualTest.java

@@ -0,0 +1,185 @@
+package com.baeldung.hibernate.entitymanager.getreference;
+
+import ch.qos.logback.classic.Level;
+import ch.qos.logback.classic.Logger;
+import org.junit.jupiter.api.*;
+import org.slf4j.LoggerFactory;
+
+import javax.persistence.EntityManager;
+import javax.persistence.EntityManagerFactory;
+import javax.persistence.Persistence;
+import java.io.ByteArrayOutputStream;
+import java.io.OutputStream;
+import java.io.PrintStream;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+/**
+ * We need to have a running PostgreSQL instance in order to execute this integration test
+ */
+public class GetReferencePostgreSQLManualTest {
+
+    private static EntityManagerFactory entityManagerFactory;
+    private static EntityManager entityManager;
+
+    private static final PrintStream SystemOut = System.out;
+    private static OutputStream output;
+
+    @BeforeAll
+    public static void setup() {
+        // close some specific loggers so that we can clearly see Hibernate: SQL queries
+        ((Logger) LoggerFactory.getLogger("org.hibernate.SQL")).setLevel(Level.OFF);
+        ((Logger) LoggerFactory.getLogger("org.hibernate.type.descriptor.sql")).setLevel(Level.OFF);
+        ((Logger) LoggerFactory.getLogger("org.hibernate.stat")).setLevel(Level.OFF);
+
+        entityManagerFactory = Persistence.createEntityManagerFactory("com.baeldung.hibernate.entitymanager.game_player_postgresql");
+    }
+
+    private void runInTransaction(Runnable task) {
+        // We create new persistence context for each test method to discard Hibernate first level cache.
+        // So that we can see the behavior of getReference() method in a non-cached environment.
+        entityManager = entityManagerFactory.createEntityManager();
+        entityManager.getTransaction().begin();
+        task.run();
+        entityManager.getTransaction().commit();
+        // In any case, we use clear() and close() to make all the managed entities detached and cleared.
+        // So, we can be sure we test always on a clear persistence context.
+        entityManager.clear();
+        entityManager.close();
+    }
+
+    @BeforeEach
+    public void beforeEach() {
+        // stubbing System.out printStream
+        output = new ByteArrayOutputStream();
+        System.setOut(new PrintStream(output));
+    }
+
+    @AfterEach
+    public void afterEach() {
+        System.setOut(SystemOut);
+        // we print to see original output after each test method
+        System.out.print(output.toString());
+    }
+
+    @AfterAll
+    public static void tearDown() {
+        entityManagerFactory.close();
+    }
+
+    @Test
+    public void whenUsingFindMethodToUpdateGame_thenExecutesSelectForGame() {
+
+        runInTransaction(() -> {
+            Game game1 = entityManager.find(Game.class, 1L);
+            game1.setName("Game Updated 1");
+
+            entityManager.persist(game1);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select game0_.id as id1_0_0_, game0_.name as name2_0_0_ from Game game0_ where game0_.id=?\n");
+        expected.append("Hibernate: update Game set name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingGetReferenceMethodToUpdateGame_thenExecutesSelectForGame() {
+
+        runInTransaction(() -> {
+            Game game1 = entityManager.getReference(Game.class, 1L);
+            game1.setName("Game Updated 2");
+
+            entityManager.persist(game1);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select game0_.id as id1_0_0_, game0_.name as name2_0_0_ from Game game0_ where game0_.id=?\n");
+        expected.append("Hibernate: update Game set name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingFindMethodToDeletePlayer_thenExecutesSelectForPlayer() {
+
+        runInTransaction(() -> {
+            Player player2 = entityManager.find(Player.class, 2L);
+            entityManager.remove(player2);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: delete from Player where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingGetReferenceMethodToDeletePlayer_thenExecutesSelectForPlayer() {
+
+        runInTransaction(() -> {
+            Player player3 = entityManager.getReference(Player.class, 3L);
+            entityManager.remove(player3);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: delete from Player where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingFindMethodToUpdatePlayersGame_thenExecutesSelectForGame() {
+
+        runInTransaction((() -> {
+            Game game1 = entityManager.find(Game.class, 1L);
+
+            Player player1 = entityManager.find(Player.class, 1L);
+            player1.setGame(game1);
+
+            entityManager.persist(player1);
+        }));
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select game0_.id as id1_0_0_, game0_.name as name2_0_0_ from Game game0_ where game0_.id=?\n");
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: update Player set game_id=?, name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+    @Test
+    public void whenUsingGetReferenceMethodToUpdatePlayersGame_thenDoesNotExecuteSelectForGame() {
+
+        runInTransaction(() -> {
+            Game game2 = entityManager.getReference(Game.class, 2L);
+
+            Player player1 = entityManager.find(Player.class, 1L);
+            player1.setGame(game2);
+
+            entityManager.persist(player1);
+        });
+
+        StringBuilder expected = new StringBuilder();
+        expected.append("Hibernate: select ");
+        expected.append("player0_.id as id1_1_0_, player0_.game_id as game_id3_1_0_, ");
+        expected.append("player0_.name as name2_1_0_, game1_.id as id1_0_1_, game1_.name as name2_0_1_ ");
+        expected.append("from Player player0_ left outer join Game game1_ on player0_.game_id=game1_.id where player0_.id=?\n");
+        expected.append("Hibernate: update Player set game_id=?, name=? where id=?\n");
+
+        assertEquals(expected.toString(), output.toString());
+    }
+
+}