Querying in One to many mappings in hibernate

Question

Let's say I have a book entity and a library entity (one library can have multiple books, but each book belongs to only one library). They are defined as follows:

public class Library { @Id private Long id; @OneToMany(mappedBy = "library") private List<Book> books = new ArrayList<>(); }

and

public class Book { @Id private Long id; @ManyToOne @JoinColumn(name="FK_Library", nullable=false) private Library library; }

I want to find all books which belong to a particular library. One way is to create a LibraryRepository as follows:

public interface LibraryRepository extends JpaRepository<Library, Long>{}

Now I have findById(Long libraryId) method that returns a Library object with a List<Book> in it.

I was wondering if it is possible to create a BookRepository and declare a method named findByLibraryId(Long libraryId) and let hibernate query the Book table by the foreign key FK_Library, without performing any joins.

Something like this:

public interface BookRepository extends JpaRepository<Book, Long> { public List<Book> findByLibraryId(Long libraryId); }

The above method findByLibraryId does perform join. I do not want it to perform a join, since all I want is find all rows which have library_id defined by foreign key relationship.

Also is the first method more preferable then second one?

Why exactly you need this ? You can do this using native query and may be using JPQL also — Eklavya
– Eklavya, Commented Aug 10, 2020 at 11:14
Using native query might bind it to specific type of schema. — JavaLearner
– JavaLearner, Commented Aug 10, 2020 at 11:17
Also, in findById method of LibraryRepository it returns Library object with List<Book>, but it does not perform join when I see the hibernate generated query. How is this possible? — JavaLearner
– JavaLearner, Commented Aug 10, 2020 at 11:19
@OneToMany by default lazy, so first fetch the library then fetch the books of the library in separte query when getter called. So 2 queries executed. — Eklavya
– Eklavya, Commented Aug 10, 2020 at 11:22

Eklavya · Accepted Answer · 2020-08-10 11:18:37Z

3

You can use @Query with JPQL to do that

@Query("SELECT b FROM Book b WHERE b.library.id = :libraryId") public List<Book> findByLibraryId(Long libraryId);

answered Aug 10, 2020 at 11:18

Eklavya

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2 Comments

JavaLearner Over a year ago

Looking from performance side, using @Query is better or using LibraryRepository's findById method?

Eklavya Over a year ago

Depends if you want only books of library then use @Query and if you want Library also then query in LibraryRepository and rather doing two query you do that using one query using @EntityGraph

Turo · Accepted Answer · 2020-08-10 11:22:59Z

You could change the @ManyToOne relationship to lazy loading.

@ManyToOne(fetch = FetchType.LAZY)

This is the default for OneToMany, thats why you see no join when you search the Library.

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