Data relation with JPA mapping

This is a demonstration project for data mapping with Spring data JPA.

Some pointers of this project:

• There is no authentication in this project as this is a demo project.
• This project demonstrate the JPA Mapping like One-to-One, Many-to-One, Many-to-Many mapping.

One-to-One Mapping:

This mapping is where one entity with directly related with exactly one another entity. This is demonstrated by the User and UserPersonalInfo entity in the project.
In the User entity the @OneToOne annotation is used for the mapping. Cascade type is set to ALL for reflecting any changes made to the User entity.

// Entity class
public class User {
	// Other properties

	@OneToOne(cascade = CascadeType.ALL)
	@JoinColumn(name = "personal_info_id", referencedColumn = "id")
	private UserPersonalInfo personalInfo;

	// Getters and setters
}

@JoinColumn annotation is used on the owning side of the relation. In My case User is the owning side. @JoinColumn is not necessary but for controll I included the annotation. This annotation defines the relation by using the other entity's "id".

On the other hand, for the UserPersonalInfo entity, I added the @OneToOne and @JsonBackReference annotation.

// Entity class
public class UserPersonalInfo {
	// Other properties

	@OneToOne(mappedBy = "personalInfo")
	@JsonBackReference
	private User user;

	// Getters and setters
}

Here the @JsonBackReference was used to avoid Jackson data bind loop error. This annotation is required if you want to return the entity class in response. I would suggest using a DTO for API response. I returned the entity class for the sake of this demonstration.

One-to-Many Mapping:

This mapping is where one entity is related to multiple other entities. This is the most commonly used relationship. This is demonstrated by the Book and Author entity in this project.
In this case the Book is the owning side of this relationship.

// Entity class
public class Book {
	@ManyToOne
	@JoinColumn(name = "author_id")
	@JsonBackReference
	private Author author;
}

// Entity class
public class Author {
	@OneToMany(mappedBy = "author", cascade = CascadeType.ALL, orphanRemoval = true)
	private List<Book> books;
}

Here the relationship is Unidirectional as we have specified the owning and reverse side of the relationship.