How to make a paginated repository call from column entity in Spring Boot JPA

Question

I have this entity model class (Book) where an author can have written multiple books.

@Entity
@Table(name="book")
public class Book {
 
    @Id
    @GeneratedValue
    private Long id;
     
    @Column(name="book_name")
    private String bookName;
     
    @Column(name="author_id")
    private Long authorId;
     
    //Setters and getters
}

In my Spring Boot project, I don't want to have an author table since there is a third part service that defines authors and their ids, how could I make a paginated repository call for all authorIds and their books?

I would want to have an endpoint that takes in (page, size) and returns a paginated list of a AuthorDTO like so:

public abstract class AuthorDTO implements Serializable {
    public abstract Long authorId();

    public abstract List<Book> books();
}

[
    {
        "authorId": 123,
        "books": [...]
    },
    ...

]

My first thought is to create a repository call not sure how we can get a page of a custom object. This is not valid below, but I would like to do something like the following.

Page<AuthorDTO> findAllBooksGroupedByAuthorId(Pageable pageable);

Answer 1

Your code seems to suggest you are trying to show the foreign key relationship in the class as an id. JPA doesn't really do that. JPA = "Java Persistence Language" i.e. you represent the relations between Java classes that mirror the database.

So in the database you may have a foreign key like 'author_id' in the book table, but in JPA/Java side it will be an "Author" class and not just a long/int.

I hope the below helps. Iv'e just slapped it on the main() of my code so it may not be perfect but I have left some comments as well.

Once you have a Page<Book> you may then want to map it to the DTO in java.

As the query is "get books by author id" we can assume that they all have the same author ID...so there is no real need to try get this projection in the database.

EDIT: Is it not at all possible to have a reference to the author from the 3rd party?

I.e. I don't know how you are populating "Book"...but could you not as you get "Book" from the 3rd party see if you have an Author entity with the books 'author_id' and not persist a new "Author" with that ID if it doesn't already exist?

In this case you can then do an AuthorRepo and simply query like:

Page<Author> findAllBy(Pageable page)

==========================================================================

Seeming as you are fetching a Page of Books by an author Id...you should really have a JPA relationship to show that:

    @Entity
    private class Book{

        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private long id;

        @Column(name = "book_name")
        private String name;

        //Technically this could be Many:Many as a book could have 2 authors? If so....@ManyToMany
        //For simplicity (and what you seem to want) Many Books have ONE author.
        @ManyToOne(fetch = FetchType.LAZY)
        private Author author;

    }

    @Entity
    private class Author{

        //ID here - omitted for clarity

        @Column(name = "authors_name")
        String name;

        //The Author has many books.
        // Mapped by shows the bi-direction relationship. You can then do 'Author.getAuthorsBooks()'
        //Lazy means it wont fetch all the books from database/(hibernate wont) when you do AuthorRepo.get()
        //and will only do the `JOIN ON Books where` if you do Author.getAuthorsBooks()
        @OneToMany(fetch = FetchType.LAZY,mappedBy = "author")
        private Set<Book> authorsBooks = new HashSet<>();
    }

    private interface AuthorRepo extends JpaRepository<Author,Long>{
        //Note the JPA syntax.
        Page<Book> findAll(Pageable pageable);
    }

EDIT: I have only written this in an empty file...so it may need tweaking or has typos etc

If you can NOT have a separate entity for some reason for Author, having to keep your entity as it currently is...I'd do 2 queries.

I feel you can either do this in various ways.

If you MUST stick with spring's Pageable:

Get the page request in the controller and make it in to a new PageRequest.of(pagenum,size)

and feed it in to do the Page query below

List<Long> getPageOfUniqueAuthorIds(Pageable pageable);

This will give a page of author Ids.

Then you want to use that List of Longs (aithorIds) to do the second query.

List<AuthorDTOProjection> getBooksAndAuthorIdsWithAuthorsIdsIn(List<Long> authorIds);

    @Entity
    @Table(name="book")
    public class Book {

        @Id
        @GeneratedValue
        private Long id;

        @Column(name="book_name")
        private String bookName;

        @Column(name="author_id")
        private Long authorId;

        //Setters and getters
    }



    private interface BookRepo extends JpaRepository<Book,Long> {

        //The countQuery is required by Spring Paging.
        //Hibernate will need to use the count query when doing paging on a native query.
        @Query(nativeQuery = true,
        value = "SELECT DISTINCT(author_id) FROM book b ",
        countQuery = "SELECT count(*) \n" +
                "FROM (SELECT DISTINCT(author_id) FROM book b) authorIds ")
        List<Long> getPageOfUniqueAuthorIds(Pageable pageable);

        //This is not paged. You want all books with the author IDs from the page query above.
      List<Book> findAllByAuthorIdIn(List<Long> authorIds);
    }

You will then have to map the Entity to the DTO in your service layer.

        @Autowired
        BookRepo bookRepo;

        //This would be from the controller method...not declared here...
        Pageable pageableFromController = PageRequest.of(0,10);

        List<Long> pageOfUniqueAuthorIds = bookRepo.getPageOfUniqueAuthorIds(pageableFromController);

        //Get All the books with Author Ids.
        List<Book> books = bookRepo.findAllByAuthorIdIn(pageOfUniqueAuthorIds);

        //Your abstract AuthorDTO.
        abstract class AuthorDTO implements Serializable {
            public abstract Long authorId();

            public abstract List<Book> books();
        }

        //Your Author DTO needs to be implemented so I made a "View".
        @AllArgsConstructor
        class AuthorView extends AuthorDTO{

            private long authorId;
            private List<Book> books;

            @Override
            public Long authorId() {
                return authorId;
            }

            @Override
            public List<Book> books() {
                return books;
            }
        }

        //Get a List of the authorIds in the List<Books>. Could also use the original Page<Long> authorIds...
        //As an author without a book is not possible in your database.
        final List<Long> authorIdsInBooks = books.stream().map(it -> it.authorId).distinct().collect(Collectors.toList());

        //Map the Ids of authors to an Impl of your abstract DTO. Personally I don't see why the AuthorDTO is abstract.
        //I'd have expected just an abstract DTO class called "DTO" or something and then AuthorDTO impl that.
        //But as the way you have it this will work. I guess you may want more impl of the  AuthorDTO so maybe leave the AuthorDTO as abstract.
        //This can be returned to client.
        final List<AuthorView> authorViews = authorIdsInBooks.stream()
                .map(authorId -> new AuthorView(
                        authorId,
                        books.stream().filter(it -> it.authorId.equals(authorId)).collect(Collectors.toList()))
                )
                .collect(Collectors.toList());