JPA One-To-Many Relationship

1.    One-To-Many Relationship 
   
2. 在  BookCategory class 設定  One-To-Many  : 

@Entity
@Table(name = "book_category")
public class BookCategory {
    private int id;
    private String name;
    private Set<Book> books;

    @OneToMany(mappedBy = "bookCategory", cascade = CascadeType.ALL)
    public Set<Book> getBooks() {
        return books;
    }

3.  建立BookCategoryRepository 繼承   JpaRepository 

import com.hellokoding.jpa.model.BookCategory;
import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.stereotype.Repository;

@Repository
public interface BookCategoryRepository extends JpaRepository<BookCategory, Integer>{
}

4.  可以使用Repository 已經實作好,常用的CRUD功能

(findOne,findAll,save....)  

        // save a couple of categories
        BookCategory categoryA = new BookCategory("Category A");
        Set bookAs = new HashSet<Book>(){{
            add(new Book("Book A1", categoryA));
            add(new Book("Book A2", categoryA));
            add(new Book("Book A3", categoryA));
        }};
        categoryA.setBooks(bookAs);

        BookCategory categoryB = new BookCategory("Category B");
        Set bookBs = new HashSet<Book>(){{
            add(new Book("Book B1", categoryB));
            add(new Book("Book B2", categoryB));
            add(new Book("Book B3", categoryB));
        }};
        categoryB.setBooks(bookBs);

        bookCategoryRepository.save(new HashSet<BookCategory>() {{
            add(categoryA);
            add(categoryB);
        }});

        // fetch all categories
        for (BookCategory bookCategory : bookCategoryRepository.findAll()) {
            logger.info(bookCategory.toString());
        }
    }

5.  完整範例 :

https://hellokoding.com/jpa-one-to-many-relationship-mapping-example-with-spring-boot-maven-and-mysql/