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Many-to-one and JoinFormula
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‎HibernateSpringBootJoinFormula/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>

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

‎HibernateSpringBootJoinFormula/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.fetchBooks();
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};
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}
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}

‎HibernateSpringBootJoinFormula/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 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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@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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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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@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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}

‎HibernateSpringBootJoinFormula/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.ManyToOne;
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import org.hibernate.annotations.JoinFormula;
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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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private int price;
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@ManyToOne(fetch = FetchType.LAZY)
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@JoinColumn(name = "author_id")
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private Author author;
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@ManyToOne(fetch = FetchType.LAZY)
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@JoinFormula("(SELECT b.id FROM book b "
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+ "WHERE b.price < price AND b.author_id = author_id "
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+ "ORDER BY b.price DESC "
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+ "LIMIT 1)")
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private Book prevBook;
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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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public int getPrice() {
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return price;
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}
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public void setPrice(int price) {
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this.price = price;
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}
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public Book getPrevBook() {
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return prevBook;
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}
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public void setPrevBook(Book prevBook) {
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this.prevBook = prevBook;
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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
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+ ", isbn=" + isbn + ", price=" + price + '}';
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}
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}

‎HibernateSpringBootJoinFormula/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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import org.springframework.transaction.annotation.Transactional;
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@Repository
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public interface BookRepository extends JpaRepository<Book, Long> {
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@Transactional(readOnly = true)
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Book findByTitle(String title);
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}

‎HibernateSpringBootJoinFormula/src/main/java/com/bookstore/service/BookstoreService.java

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package com.bookstore.service;
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import com.bookstore.repository.BookRepository;
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import com.bookstore.entity.Book;
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import org.springframework.stereotype.Service;
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import org.springframework.transaction.annotation.Transactional;
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@Service
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public class BookstoreService {
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private final BookRepository bookRepository;
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public BookstoreService(BookRepository bookRepository) {
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this.bookRepository = bookRepository;
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}
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@Transactional(readOnly = true)
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public void fetchBooks() {
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Book book = bookRepository.findById(4L).orElseThrow();
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Book prevBook = book.getPrevBook();
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System.out.println("Fetched book with id 4: " + book);
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System.out.println("Fetched book with next smallest price: " + prevBook);
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}
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}

‎HibernateSpringBootJoinFormula/src/main/resources/application.properties

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spring.datasource.url=jdbc:mysql://localhost:3306/bookstoredb?createDatabaseIfNotExist=true
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spring.datasource.username=root
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spring.datasource.password=root
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spring.jpa.hibernate.ddl-auto=create
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spring.jpa.show-sql=true
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spring.jpa.properties.hibernate.dialect=org.hibernate.dialect.MySQL8Dialect
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spring.jpa.open-in-view=false
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spring.datasource.initialization-mode=always
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spring.datasource.platform=mysql
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logging.level.org.hibernate.type.descriptor.sql=TRACE

‎HibernateSpringBootJoinFormula/src/main/resources/data-mysql.sql

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insert into author (age, name, genre, id) values (23, "Mark Janel", "Anthology", 1);
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insert into author (age, name, genre, id) values (43, "Olivia Goy", "Horror", 2);
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insert into author (age, name, genre, id) values (51, "Quartis Young", "Anthology", 3);
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insert into author (age, name, genre, id) values (34, "Joana Nimar", "History", 4);
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insert into book (isbn, title, price, author_id, id) values ("001-JN", "A History of Ancient Prague", 23, 4, 1);
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insert into book (isbn, title, price, author_id, id) values ("002-JN", "A People's History", 34, 4, 2);
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insert into book (isbn, title, price, author_id, id) values ("001-MJ", "The Beatles Anthology", 55, 1, 3);
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insert into book (isbn, title, price, author_id, id) values ("002-MJ", "Anthology Of '99", 44, 1, 4);
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insert into book (isbn, title, price, author_id, id) values ("001-OG", "Carrie", 33, 2, 5);
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insert into book (isbn, title, price, author_id, id) values ("002-OG", "Last Day", 25, 2, 6);
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insert into book (isbn, title, price, author_id, id) values ("003-JN", "History Today", 41, 4, 7);
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insert into book (isbn, title, price, author_id, id) values ("003-MJ", "Anthology Of A Game", 21, 1, 8);

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