[FIXED] I want to search book(Many) entity from library(One) entity. (A->B, B->A)

Issue

It's so working well when I was trying to search "nice_library" from "book". I guess because the nice_library FK is saved on the book.

book = bookRepository.findById(book.getId()).orElseThrow(() -> new RuntimeException());
Long id = book.getNiceLibrary().getId();
assertEquals(id, niceLibrary.getId());

but I can't get anything when I tried to search "book" from "nice_library". I guess because nice_library doesn't have FK of book

niceLibrary = niceLibraryRepository.findById(niceLibrary.getId()).orElseThrow(() -> new RuntimeException());
List<Book> books = niceLibrary.getBooks(); // WHAT THE

I was expecting the "Book.id" to be saved on the "nice_library" table for search to "book" from "nice_library".

Book.java

package com.example.jpa;

import lombok.Getter;
import lombok.Setter;

import javax.persistence.*;

@Getter
@Entity
public class Book {
    @Id
    @GeneratedValue
    @Column
    private Long id;

    @Setter
    @ManyToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "LIBRARY_ID") // Foreign Key
    private NiceLibrary niceLibrary;
}

NiceLibrary.java

package com.example.jpa;

import lombok.Getter;

import javax.persistence.*;
import java.util.ArrayList;
import java.util.List;

@Getter
@Entity
public class NiceLibrary {
    @Id
    @GeneratedValue
    @Column
    private Long id;

    @OneToMany(mappedBy = "niceLibrary")
    private List<Book> books = new ArrayList<>();
}

book table

id|library_id|
--+----------+
77|        78|

nice_library

id|
--+
78|

Test Code

@SpringBootTest
class BookServiceTest {
    @Autowired
    private BookRepository bookRepository;
    @Autowired
    private NiceLibraryRepository niceLibraryRepository;

    @Test
    public void book() {
        Book book = new Book();
        NiceLibrary niceLibrary = new NiceLibrary();

        niceLibrary.getBooks().add(book);
        book.setNiceLibrary(niceLibrary);
        bookRepository.save(book);

        book = bookRepository.findById(book.getId()).orElseThrow(() -> new RuntimeException());
        Long id = book.getNiceLibrary().getId();
        assertEquals(id, niceLibrary.getId());

        niceLibrary = niceLibraryRepository.findById(niceLibrary.getId()).orElseThrow(() -> new RuntimeException());
        List<Book> books = niceLibrary.getBooks(); // ???
    }

How can I search book from nice_library?

Solution

From what I understand you want to be able to get a specific book from the library.

The one-to-many relationship is setup by having a reference to the library in the book table.

There is no reason to store the book's id in the library table.

This is all you need to setup a one-to-many relationship and you have this correct.

+-----------------+
| Book            |
+-----------------+
| id              |
| LIBRARY_ID (fk) |
+-----------------+

+------------+
| Library    |
+------------+
| id         |
+------------+

The way you are describing the problem is that you are expecting a library to only have one book, which is about as big as my library.

In a one-to-many relationship the one side (library in this case) will always have a collection of entities and the other side (book in this case) will only have a reference to a single entity.

Answered By - TRWP
Answer Checked By - Terry (JavaFixing Volunteer)