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DTO via a custom ResultTransformer
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‎HibernateSpringBootDtoCustomResultTransformer/README.md

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**[How To Fetch DTO Via `ResultTransformer` and JPQL](https://github.com/AnghelLeonard/Hibernate-SpringBoot/tree/master/HibernateSpringBootDtoResultTransformerJpql)**
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**Description:** Fetching more data than needed is prone to performance penalties. Using DTO allows us to extract only the needed data. In this application we rely on Hibernate, `ResultTransformer` and JPQL.
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**Key points:**
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- use `AliasToBeanConstructorResultTransformer` for DTO without setters, with constructor
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- use `Transformers.aliasToBean()` for DTO with setters
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- use `EntityManager.createQuery()` and `unwrap(org.hibernate.query.Query.class)`
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- starting with Hibernate 5.2, `ResultTransformer` is deprecated, but until a replacement will be available (in Hibernate 6.0) it can be used ([read further](https://discourse.hibernate.org/t/hibernate-resulttransformer-is-deprecated-what-to-use-instead/232))
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- for using Spring Data Projections check this [item](https://github.com/AnghelLeonard/Hibernate-SpringBoot/tree/master/HibernateSpringBootDtoViaProjections)
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<a href="https://leanpub.com/java-persistence-performance-illustrated-guide"><p align="center"><img src="https://github.com/AnghelLeonard/Hibernate-SpringBoot/blob/master/Java%20Persistence%20Performance%20Illustrated%20Guide.jpg" height="410" width="350"/></p></a>

‎HibernateSpringBootDtoCustomResultTransformer/pom.xml

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<?xml version="1.0" encoding="UTF-8"?>
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<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
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xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
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<modelVersion>4.0.0</modelVersion>
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<groupId>com.jpa</groupId>
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<artifactId>HibernateSpringBootDtoCustomResultTransformer</artifactId>
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<version>1.0</version>
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<packaging>jar</packaging>
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<name>HibernateSpringBootDtoCustomResultTransformer</name>
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<description>JPA project for Spring Boot</description>
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<parent>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-parent</artifactId>
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<version>2.1.4.RELEASE</version>
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<relativePath/> <!-- lookup parent from repository -->
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</parent>
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<properties>
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<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
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<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
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<java.version>1.8</java.version>
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</properties>
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<dependencies>
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-data-jpa</artifactId>
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</dependency>
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-jdbc</artifactId>
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</dependency>
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-web</artifactId>
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</dependency>
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<dependency>
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<groupId>mysql</groupId>
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<artifactId>mysql-connector-java</artifactId>
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<scope>runtime</scope>
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</dependency>
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<dependency>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-starter-test</artifactId>
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<scope>test</scope>
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</dependency>
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</dependencies>
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<build>
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<plugins>
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<plugin>
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<groupId>org.springframework.boot</groupId>
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<artifactId>spring-boot-maven-plugin</artifactId>
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</plugin>
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</plugins>
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</build>
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</project>

‎HibernateSpringBootDtoCustomResultTransformer/src/main/java/com/bookstore/MainApplication.java

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package com.bookstore;
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import com.bookstore.service.BookstoreService;
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import org.springframework.boot.ApplicationRunner;
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import org.springframework.boot.SpringApplication;
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import org.springframework.boot.autoconfigure.SpringBootApplication;
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import org.springframework.context.annotation.Bean;
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@SpringBootApplication
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public class MainApplication {
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private final BookstoreService bookstoreService;
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public MainApplication(BookstoreService bookstoreService) {
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this.bookstoreService = bookstoreService;
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}
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public static void main(String[] args) {
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SpringApplication.run(MainApplication.class, args);
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}
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@Bean
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public ApplicationRunner init() {
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return args -> {
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bookstoreService.fetchAuthorWithBook();
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};
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}
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}

‎HibernateSpringBootDtoCustomResultTransformer/src/main/java/com/bookstore/dao/Dao.java

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package com.bookstore.dao;
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import com.bookstore.dto.AuthorDto;
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import com.bookstore.transformer.AuthorBookTransformer;
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import java.util.List;
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import javax.persistence.EntityManager;
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import javax.persistence.PersistenceContext;
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import javax.persistence.Query;
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import org.springframework.stereotype.Repository;
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import org.springframework.transaction.annotation.Transactional;
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@Repository
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public class Dao {
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@PersistenceContext
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private EntityManager entityManager;
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@Transactional(readOnly = true)
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public List<AuthorDto> fetchAuthorWithBook() {
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Query query = entityManager
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.createNativeQuery(
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"SELECT a.id AS author_id, a.name AS name, a.age AS age, "
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+ "b.id AS book_id, b.title AS title "
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+ "FROM author a JOIN book b ON a.id=b.author_id")
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.unwrap(org.hibernate.query.NativeQuery.class)
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.setResultTransformer(new AuthorBookTransformer());
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List<AuthorDto> authors = query.getResultList();
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return authors;
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}
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}

‎HibernateSpringBootDtoCustomResultTransformer/src/main/java/com/bookstore/dto/AuthorDto.java

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package com.bookstore.dto;
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import java.io.Serializable;
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import java.util.ArrayList;
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import java.util.List;
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public class AuthorDto implements Serializable {
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private static final long serialVersionUID = 1L;
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private Long authorId;
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private String name;
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private int age;
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private List<BookDto> books = new ArrayList<>();
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public AuthorDto() {
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}
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public AuthorDto(Long authorId, String name, int age) {
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this.authorId = authorId;
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this.name = name;
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this.age = age;
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}
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public Long getId() {
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return authorId;
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}
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public void setId(Long authorId) {
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this.authorId = authorId;
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}
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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public int getAge() {
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return age;
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}
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public void setAge(int age) {
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this.age = age;
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}
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public List<BookDto> getBooks() {
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return books;
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}
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public void setBooks(List<BookDto> books) {
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this.books = books;
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}
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public void addBook(BookDto book) {
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books.add(book);
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}
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@Override
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public String toString() {
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return "AuthorDto{" + "authorId=" + authorId + ", name=" + name + ", age=" + age + '}';
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}
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}

‎HibernateSpringBootDtoCustomResultTransformer/src/main/java/com/bookstore/dto/BookDto.java

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package com.bookstore.dto;
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import java.io.Serializable;
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public class BookDto implements Serializable {
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private static final long serialVersionUID = 1L;
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private Long bookId;
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private String title;
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public BookDto() {}
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public BookDto(Long bookId, String title) {
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this.bookId = bookId;
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this.title = title;
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}
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public Long getId() {
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return bookId;
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}
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public void setId(Long bookId) {
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this.bookId = bookId;
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}
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public String getTitle() {
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return title;
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}
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public void setTitle(String title) {
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this.title = title;
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}
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@Override
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public String toString() {
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return "BookDto{" + "bookId="
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+ bookId + ", title=" + title + '}';
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}
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}

‎HibernateSpringBootDtoCustomResultTransformer/src/main/java/com/bookstore/entity/Author.java

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package com.bookstore.entity;
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import java.io.Serializable;
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import java.util.ArrayList;
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import java.util.Iterator;
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import java.util.List;
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import javax.persistence.CascadeType;
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import javax.persistence.Entity;
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import javax.persistence.GeneratedValue;
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import javax.persistence.GenerationType;
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import javax.persistence.Id;
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import javax.persistence.OneToMany;
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@Entity
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public class Author implements Serializable {
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private static final long serialVersionUID = 1L;
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@Id
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@GeneratedValue(strategy = GenerationType.IDENTITY)
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private Long id;
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private String name;
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private String genre;
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private int age;
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@OneToMany(cascade = CascadeType.ALL,
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mappedBy = "author", orphanRemoval = true)
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private List<Book> books = new ArrayList<>();
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public void addBook(Book book) {
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this.books.add(book);
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book.setAuthor(this);
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}
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public void removeBook(Book book) {
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book.setAuthor(null);
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this.books.remove(book);
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}
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public void removeBooks() {
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Iterator<Book> iterator = this.books.iterator();
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while (iterator.hasNext()) {
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Book book = iterator.next();
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book.setAuthor(null);
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iterator.remove();
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}
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}
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public Long getId() {
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return id;
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}
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public void setId(Long id) {
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this.id = id;
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}
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public String getName() {
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return name;
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}
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public void setName(String name) {
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this.name = name;
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}
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public String getGenre() {
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return genre;
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}
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public void setGenre(String genre) {
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this.genre = genre;
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}
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public int getAge() {
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return age;
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}
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public void setAge(int age) {
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this.age = age;
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}
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public List<Book> getBooks() {
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return books;
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}
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public void setBooks(List<Book> books) {
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this.books = books;
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}
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@Override
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public String toString() {
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return "Author{" + "id=" + id + ", name=" + name
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+ ", genre=" + genre + ", age=" + age + '}';
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}
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}

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