@Formula to count object from a @OneToMany association in spring data jpa

Question

In a java rest api, with spring boot, and spring-dta-jpa, db is postgresql I have a Book entity that contains bookCopies . These bookCopies can be available or not. I'm trying to retrieve the book with the numbers of copies available. The books can be searched by title or author name.

I manage to get the info by adding a @Transient field in my book entity, with the @Transient annotation on a getNbCopiesAvailable() method, but I'm asked to do it with @Formula annotation, and I can't figure out how to do it.

I'm actually getting an error:

org.postgresql.util.PSQLException: ERREUR: plus d'une ligne renvoyée par une sous-requête utilisée comme une expression 
// which means that several lines are sent by the sub-query

Here are my entities:

@Entity
@Getter
@Setter
@NoArgsConstructor
public class Book {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String isbn;
    @NotNull
    private String title;

    @JsonIgnore
    @OneToMany(mappedBy = "book", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<BookCopy> copyList = new ArrayList<>();

    @NotNull
    @ManyToMany(cascade = {CascadeType.PERSIST, CascadeType.MERGE})
    @JoinTable(name = "book_author",
               joinColumns = @JoinColumn(name = "book_id"),
               inverseJoinColumns = @JoinColumn(name = "author_id"))
    private Set<Author> authors = new HashSet<>();

    @Formula("(SELECT COUNT(bc.id) FROM book b left join book_copy bc on bc.book_id = b.id  WHERE bc.available = 'true' GROUP BY b.id)")
    private Long nbCopiesAvailable;


@Entity
@Getter
@Setter
@Builder
@AllArgsConstructor
public class BookCopy {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String barcode;
    private String editor;
    private boolean available = true;

    @ManyToOne(fetch = FetchType.LAZY)
    private Book book;

Here is my repository with the query to retrieve books by title and author:

    @Query("select b, a from Book b join b.authors a where b.title like %:title% and concat(a.firstName, ' ', a.lastName) like %:author%")
    List<Book> findByTitleAndAuthor(@Param("title")String title,@Param("author") String author);

The @Formula("(SELECT COUNT(bc.id) FROM book b left join book_copy bc on bc.book_id = b.id WHERE bc.available = 'true' GROUP BY b.id)") was hard to write without having syntax errors, and it's strange to me to mix native sql and jpql. But if I try to write the @Formula in jpql it doesn't work at all.

I checked this topic that was the closest to my problem ( @Formula count on ManyToMany ), but it still doesn't work.

Answer 1

You need to reference the book in the where clause:

@Formula("(SELECT COUNT(bc.id) FROM book b " +
         "left join book_copy bc on bc.book_id = b.id " +
         "WHERE bc.available = 'true' " +
         "and b.id = id " + // This is the important condititon
         "GROUP BY b.id)")
private Long nbCopiesAvailable;

Otherwise your query is returning all books.