[BAEL-4056] Guide to MultipleBagFetchException (#10304)

* Removes DummyEntity
* Uses Artist Entity instead of DummyEntity

persistence-modules/java-jpa-3/src/main/java/com/baeldung/jpa/multiplebagfetchexception/DummyEntity.java

@@ -1,43 +0,0 @@
-package com.baeldung.jpa.multiplebagfetchexception;
-
-import javax.persistence.ElementCollection;
-import javax.persistence.Entity;
-import javax.persistence.GeneratedValue;
-import javax.persistence.GenerationType;
-import javax.persistence.Id;
-import java.util.List;
-import java.util.Objects;
-
-@Entity
-class DummyEntity {
-
-    @Id
-    @GeneratedValue(strategy = GenerationType.AUTO)
-    private Long id;
-
-    @ElementCollection
-    private List<String> collection1;
-
-    @ElementCollection
-    private List<String> collection2;
-
-    @Override
-    public boolean equals(Object o) {
-        if (this == o)
-            return true;
-        if (o == null || getClass() != o.getClass())
-            return false;
-
-        DummyEntity that = (DummyEntity) o;
-
-        return Objects.equals(id, that.id);
-    }
-
-    @Override
-    public int hashCode() {
-        return id != null ? id.hashCode() : 0;
-    }
-
-    protected DummyEntity() {
-    }
-}

persistence-modules/java-jpa-3/src/main/resources/META-INF/persistence.xml

@@ -57,7 +57,6 @@
     </persistence-unit>
     <persistence-unit name="jpa-h2-multiple-bag-fetch-exception">
         <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
-        <class>com.baeldung.jpa.multiplebagfetchexception.DummyEntity</class>
         <class>com.baeldung.jpa.multiplebagfetchexception.Album</class>
         <class>com.baeldung.jpa.multiplebagfetchexception.Song</class>
         <class>com.baeldung.jpa.multiplebagfetchexception.User</class>

persistence-modules/java-jpa-3/src/test/java/com/baeldung/jpa/multiplebagfetchexception/MultipleBagFetchExceptionIntegrationTest.java

@@ -29,9 +29,9 @@ public class MultipleBagFetchExceptionIntegrationTest {
     public void whenFetchingMoreThanOneBag_thenThrowAnException() {
         IllegalArgumentException exception =
           assertThrows(IllegalArgumentException.class, () -> {
-            String jpql = "SELECT dummy FROM DummyEntity dummy "
+            String jpql = "SELECT artist FROM Artist artist "
-              + "JOIN FETCH dummy.collection1 "
+              + "JOIN FETCH artist.songs "
-              + "JOIN FETCH dummy.collection2 ";
+              + "JOIN FETCH artist.offers ";
 
             entityManager.createQuery(jpql);
         });