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Composite PK via Embeddable
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‎HibernateSpringBootCompositeKeyEmbeddable/README.md

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**[OneToMany Bidirectional](https://github.com/AnghelLeonard/Hibernate-SpringBoot/tree/master/HibernateSpringBootOneToManyBidirectional)**
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**Description:** This application is a proof of concept of how is correct to implement the bidirectional `@OneToMany` association.
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**Key points:**\
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- always cascade from parent to child\
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- use `mappedBy` on the parent\
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- use `orphanRemoval` on parent in order to remove children without references\
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- use helper methods on parent to keep both sides of the association in sync\
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- use lazy fetching on both side of the association\
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- as entities identifiers, use assigned identifiers (business key, natural key (`@NaturalId`)) and/or database-generated identifiers and override (on child-side) properly the `equals()` and `hashCode()` methods as [here](https://vladmihalcea.com/the-best-way-to-implement-equals-hashcode-and-tostring-with-jpa-and-hibernate/)\
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- if `toString()` need to be overridden, then pay attention to involve only for the basic attributes fetched when the entity is loaded from the database
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**Note:** Pay attention to remove operations, especially to removing child entities. The `CascadeType.REMOVE` and `orphanRemoval=true` may produce too many queries. Relying on *bulk* operations is most of the time the best way to go for deletions.
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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>

‎HibernateSpringBootCompositeKeyEmbeddable/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>HibernateSpringBootCompositeKeyEmbeddable</artifactId>
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<version>1.0</version>
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<packaging>jar</packaging>
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<name>HibernateSpringBootCompositeKeyEmbeddable</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>

‎HibernateSpringBootCompositeKeyEmbeddable/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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System.out.println("\nAdd author with books ...");
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bookstoreService.addAuthorWithBooks();
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System.out.println("\nFetch author by name ...");
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bookstoreService.fetchAuthorByName();
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System.out.println("\nRemove a book of an author ...");
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bookstoreService.removeBookOfAuthor();
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System.out.println("\nRemove author (and books via cascade) ...");
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bookstoreService.removeAuthor();
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};
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}
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}

‎HibernateSpringBootCompositeKeyEmbeddable/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.EmbeddedId;
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import javax.persistence.Entity;
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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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@EmbeddedId
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private AuthorId id;
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private String genre;
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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 AuthorId getId() {
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return id;
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}
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public void setId(AuthorId id) {
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this.id = id;
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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 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 + ", genre=" + genre + '}';
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}
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}

‎HibernateSpringBootCompositeKeyEmbeddable/src/main/java/com/bookstore/entity/AuthorId.java

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package com.bookstore.entity;
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import java.io.Serializable;
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import java.util.Objects;
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import javax.persistence.Column;
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import javax.persistence.Embeddable;
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@Embeddable
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public class AuthorId implements Serializable {
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private static final long serialVersionUID = 1L;
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@Column(name = "name")
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private String name;
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@Column(name = "age")
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private int age;
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public AuthorId() {
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}
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public AuthorId(String name, int age) {
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this.name = name;
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this.age = age;
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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 int getAge() {
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return age;
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}
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@Override
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public int hashCode() {
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int hash = 3;
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hash = 23 * hash + Objects.hashCode(this.name);
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hash = 23 * hash + this.age;
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return hash;
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}
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@Override
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public boolean equals(Object obj) {
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if (this == obj) {
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return true;
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}
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if (obj == null) {
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return false;
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}
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if (getClass() != obj.getClass()) {
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return false;
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}
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final AuthorId other = (AuthorId) obj;
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if (this.age != other.age) {
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return false;
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}
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if (!Objects.equals(this.name, other.name)) {
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return false;
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}
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return true;
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}
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@Override
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public String toString() {
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return "AuthorId{" + "name=" + name + ", age=" + age + '}';
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}
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}

‎HibernateSpringBootCompositeKeyEmbeddable/src/main/java/com/bookstore/entity/Book.java

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package com.bookstore.entity;
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import java.io.Serializable;
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import javax.persistence.Entity;
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import javax.persistence.FetchType;
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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.JoinColumn;
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import javax.persistence.JoinColumns;
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import javax.persistence.ManyToOne;
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@Entity
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public class Book 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 title;
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private String isbn;
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@ManyToOne(fetch = FetchType.LAZY)
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@JoinColumns({
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@JoinColumn(
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name = "name",
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referencedColumnName = "name"),
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@JoinColumn(
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name = "age",
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referencedColumnName = "age")
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})
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private Author author;
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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 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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public String getIsbn() {
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return isbn;
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}
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public void setIsbn(String isbn) {
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this.isbn = isbn;
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}
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public Author getAuthor() {
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return author;
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}
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public void setAuthor(Author author) {
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this.author = author;
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}
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@Override
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public boolean equals(Object obj) {
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if (this == obj) {
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return true;
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}
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if (getClass() != obj.getClass()) {
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return false;
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}
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return id != null && id.equals(((Book) obj).id);
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}
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@Override
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public int hashCode() {
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return 2021;
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}
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@Override
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public String toString() {
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return "Book{" + "id=" + id + ", title=" + title + ", isbn=" + isbn + '}';
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}
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}

‎HibernateSpringBootCompositeKeyEmbeddable/src/main/java/com/bookstore/repository/AuthorRepository.java

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package com.bookstore.repository;
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import com.bookstore.entity.Author;
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import com.bookstore.entity.AuthorId;
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import org.springframework.data.jpa.repository.JpaRepository;
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import org.springframework.data.jpa.repository.Query;
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import org.springframework.stereotype.Repository;
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@Repository
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public interface AuthorRepository extends JpaRepository<Author, AuthorId> {
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@Query("SELECT a FROM Author a "
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+ "JOIN FETCH a.books WHERE a.id = ?1")
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public Author fetchWithBooks(AuthorId id);
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@Query("SELECT a FROM Author a WHERE a.id.name = ?1")
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public Author fetchByName(String name);
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}

‎HibernateSpringBootCompositeKeyEmbeddable/src/main/java/com/bookstore/repository/BookRepository.java

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package com.bookstore.repository;
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import com.bookstore.entity.Book;
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import org.springframework.data.jpa.repository.JpaRepository;
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import org.springframework.stereotype.Repository;
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@Repository
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public interface BookRepository extends JpaRepository<Book, Long> {
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}

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