Why relationships matter in JPA in Spring Boot - See It in Action

Understanding Why Relationships Matter in JPA

📖 Scenario: You are building a simple Spring Boot application to manage a library. The library has authors and books. Each book is written by one author, and each author can write many books.To keep the data organized and connected, you need to use relationships in JPA (Java Persistence API). This helps the application understand how authors and books relate to each other.

🎯 Goal: Build two JPA entity classes, Author and Book, and connect them using a OneToMany and ManyToOne relationship. This will show how JPA manages related data automatically.

📋 What You'll Learn

Create an Author entity with id and name fields

Create a Book entity with id, title, and a reference to Author

Set up a OneToMany relationship from Author to Book

Set up a ManyToOne relationship from Book to Author

💡 Why This Matters

🌍 Real World

Managing related data like authors and books is common in library systems, online stores, and content management. Relationships help keep data consistent and easy to query.

💼 Career

Understanding JPA relationships is essential for backend developers working with databases in Java applications, especially in Spring Boot projects.

1

Create the Author entity

Create a JPA entity class called Author with fields id (annotated with @Id and @GeneratedValue) and name (type String). Use @Entity annotation on the class.

Spring Boot

@Entity
public class Author {
    // Your code here
}

Need a hint?

Remember to import javax.persistence.Entity, javax.persistence.Id, and javax.persistence.GeneratedValue.

2

Create the Book entity with author reference

Create a JPA entity class called Book with fields id (annotated with @Id and @GeneratedValue), title (type String), and a field author of type Author annotated with @ManyToOne. Use @Entity annotation on the class.

Spring Boot

@Entity
public class Author {
    @Id
    @GeneratedValue
    private Long id;
    private String name;

    // Getters and setters
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
}

@Entity
public class Book {
    // Your code here
}

Need a hint?

Don't forget to import javax.persistence.ManyToOne for the author field.

3

Add OneToMany relationship in Author

In the Author class, add a field books of type List<Book> annotated with @OneToMany(mappedBy = "author"). Initialize it with an empty ArrayList. Add getter and setter for books.

Spring Boot

@Entity
public class Author {
    @Id
    @GeneratedValue
    private Long id;
    private String name;

    // Getters and setters
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }

    // Your code here
}

@Entity
public class Book {
    @Id
    @GeneratedValue
    private Long id;
    private String title;

    @ManyToOne
    private Author author;

    // Getters and setters
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getTitle() {
        return title;
    }
    public void setTitle(String title) {
        this.title = title;
    }
    public Author getAuthor() {
        return author;
    }
    public void setAuthor(Author author) {
        this.author = author;
    }
}

Need a hint?

Use mappedBy = "author" to link to the author field in Book.

4

Complete the relationship setup

Ensure both Author and Book classes have proper imports for JPA annotations and collections. Confirm the Author class has @OneToMany(mappedBy = "author") on books and the Book class has @ManyToOne on author. This completes the bidirectional relationship.

Spring Boot

import java.util.ArrayList;
import java.util.List;
import javax.persistence.Entity;
import javax.persistence.Id;
import javax.persistence.GeneratedValue;
import javax.persistence.OneToMany;

@Entity
public class Author {
    @Id
    @GeneratedValue
    private Long id;
    private String name;

    @OneToMany(mappedBy = "author")
    private List books = new ArrayList();

    // Getters and setters
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public List getBooks() {
        return books;
    }
    public void setBooks(List books) {
        this.books = books;
    }
}

@Entity
public class Book {
    @Id
    @GeneratedValue
    private Long id;
    private String title;

    @ManyToOne
    private Author author;

    // Getters and setters
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }
    public String getTitle() {
        return title;
    }
    public void setTitle(String title) {
        this.title = title;
    }
    public Author getAuthor() {
        return author;
    }
    public void setAuthor(Author author) {
        this.author = author;
    }
}

// Your code here to confirm imports and annotations

Need a hint?

Check that all necessary imports and annotations are present for the relationship to work.