When there are multiple many-to-many relations with {{FetchMode.JOIN}}, {{session.find}} returns a result with a child relation of type {{List}} containing duplicates.
To avoid {{MultipleBagFetchException: cannot simultaneously fetch multiple bags}} when multiple {{@ManyToMany}} collections have {{@Fetch(FetchMode.JOIN)}}, only one collection has type {{List}} and others have type {{Set}}.
{code:java}@Entity
@Data
public class Book implements Serializable {
@Id
@GeneratedValue
private Long id;
private String isbn;
private String title;
private LocalDate publicationDate;
@ManyToMany
@Fetch(FetchMode.JOIN)
private List<Author> authors = new ArrayList<>();
@ManyToMany
@Fetch(FetchMode.JOIN)
private Set<Category> categories = new LinkedHashSet<>();
}{code}
With the following test data
{code:java}softwareDevelopment = new Category("Software development");
session.save(softwareDevelopment);
systemDesign = new Category("System design");
session.save(systemDesign);
martinFowler = new Author("Martin Fowler");
session.save(martinFowler);
gregorHohpe = new Author("Gregor Hohpe");
session.save(gregorHohpe);
gregorHohpe = new Author();
gregorHohpe.setFullName("Gregor Hohpe");
session.save(gregorHohpe);
bobbyWoolf = new Author();
bobbyWoolf.setFullName("Bobby Woolf");
session.save(bobbyWoolf);
poeaa = new Book();
poeaa.setIsbn("007-6092019909");
poeaa.setTitle("Patterns of Enterprise Application Architecture");
poeaa.setPublicationDate(LocalDate.parse("2002-11-15"));
poeaa.getAuthors().addAll(List.of(martinFowler));
poeaa.getCategories().addAll(List.of(softwareDevelopment, systemDesign));
session.save(poeaa);
eip = new Book();
eip.setIsbn("978-0321200686");
eip.setTitle("Enterprise Integration Patterns");
eip.setPublicationDate(LocalDate.parse("2003-10-20"));
eip.getAuthors().addAll(List.of(gregorHohpe, bobbyWoolf));
eip.getCategories().addAll(List.of(softwareDevelopment, systemDesign));
session.save(eip);{code}
a Book entity found by ID contains duplicates in {{List<Author> authors}}
{code:java}@Test
void testFindByIdNoTrans() {
try (Session session = HibernateUtil.getSessionFactory().openSession()) {
Book poeaa = session.find(Book.class, this.poeaa.getId());
Book eip = session.find(Book.class, this.eip.getId());
assertThat(poeaa.getTitle()).isEqualTo(this.poeaa.getTitle());
assertThatHasAuthors(poeaa, martinFowler.getFullName());
assertThat(eip.getTitle()).isEqualTo(this.eip.getTitle());
assertThatHasAuthors(eip, gregorHohpe.getFullName(), bobbyWoolf.getFullName());
}
}{code}
{code:java}List<Author> authors = ["Martin Fowler", "Martin Fowler"]{code}
Probably, because {{categories}} has size 2:
{code:java}Set<Category> categories = ["Software development", "System design"]{code}
When there are 2 authors and 2 categories there are no duplicates in child relations.
Beginning transaction with {{session.beginTransaction()}} doesn't have any effect on the duplicates.
See the test [ com https://github . com/evgeniy-khist/hibernate-fetch-mode-join/blob/master/src/test/java/com/ example . / hibernate . / BookFetchModeJoinWithSetTests .java |https://github.com/evgeniy-khist/hibernate-fetch-mode-join/blob/master/src/test/java/com/example/hibernate/BookFetchModeJoinWithSetTests.java |smart-link ].
Using Spring Data JPA and making the test transactional with {{@org.springframework.transaction.annotation.Transactional}} solves the problem with duplicates. Spring Boot version - 2.2.1.RELEASE, Spring Data JPA version - 2.2.1.RELEASE, Hibernate version - 5.4.8.Final.
See the test [https://github.com/evgeniy-khist/ hibernate spring - data-jpa- fetch-mode-join/blob/master/src/test/java/com/example/ hibernate spring / data/jpa/ BookFetchModeJoinWithSetTests.java|https://github.com/evgeniy-khist/ hibernate spring - data-jpa- fetch-mode-join/blob/master/src/test/java/com/example/ hibernate spring / data/jpa/ BookFetchModeJoinWithSetTests.java|smart-link] . |
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