When building applications with Spring Boot and JPA (Java Persistence API), managing relationships between entities is crucial. Understanding how to map these relationships effectively will help you model your data accurately and perform efficient queries. In this guide, we'll explore the different types of JPA mappings supported by Spring Boot: one-to-one, one-to-many, many-to-one, and many-to-many. Will provide example on both unidirectional and bidirectional mappings.
In a unidirectional one-to-one relationship, one entity references another without the second entity knowing about the first.
Example:
// User.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.OneToOne; @Data @Entity public class User { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String username; @OneToOne(cascade = CascadeType.ALL) @JoinColumn(name = "fk_profile_id", referencedColumnName = "id") private Profile profile; } // Profile.java import javax.persistence.Entity; import javax.persistence.Id; @Data @Entity public class Profile { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String bio; }
In a bidirectional one-to-one relationship, both entities reference to each other.
Example:
// User.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.OneToOne; @Data @Entity public class User { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String username; @OneToOne(mappedBy = "user", cascade = CascadeType.ALL) private Profile profile; } // Profile.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.OneToOne; import javax.persistence.JoinColumn; @Data @Entity public class Profile { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String bio; @OneToOne @JoinColumn(name = "user_id") private User user; }
In a unidirectional one-to-many relationship, one entity references a collection of another entity.
Example:
// Author.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.OneToMany; import java.util.List; @Entity @Data public class Author { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String name; @OneToMany(cascade = CascadeType.ALL) @JoinColumn(name = "fk_book_id", referencedColumnName = "id") private List<Book> books; } // Book.java import javax.persistence.Entity; import javax.persistence.Id; @Entity @Data public class Book { @Id private Long id; private String title; }
In a bidirectional one-to-many relationship, both entities are referenced to each other.
Example:
// Author.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.OneToMany; import javax.persistence.CascadeType; import javax.persistence.FetchType; import java.util.List; @Entity @Data public class Author { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String name; @OneToMany(mappedBy = "author", cascade = CascadeType.ALL, fetch = FetchType.EAGER) private List<Book> books; } // Book.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.ManyToOne; @Entity @Data public class Book { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String title; @ManyToOne(name = "fk_author_id", referencedColumnName = "id") private Author author; }
In a unidirectional many-to-one relationship, multiple entities reference a single entity.
Example:
// Order.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.ManyToOne; @Entity @Data public class Order { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String description; @ManyToOne private Customer customer; } // Customer.java import javax.persistence.Entity; import javax.persistence.Id; @Entity @Data public class Customer { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String name; }
In a bidirectional many-to-one relationship, the referenced entity has a collection of the referring entities.
Example:
// Order.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.ManyToOne; @Entity @Data public class Order { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String description; @ManyToOne private Customer customer; } // Customer.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.OneToMany; import java.util.List; @Entity @Data public class Customer { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String name; @OneToMany(mappedBy = "customer") private List<Order> orders; }
In a unidirectional many-to-many relationship, each entity has a collection of the other, but the relationship is not explicitly managed by either side.
Example:
// Student.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.ManyToMany; import java.util.Set; @Entity @Data public class Student { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String name; @ManyToMany private Set<Course> courses; } // Course.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.ManyToMany; import java.util.Set; @Entity @Data public class Course { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String title; @ManyToMany private Set<Student> students; }
In a bidirectional many-to-many relationship, both entities are referencing each other, and the relationship is explicitly managed.
Example:
// Student.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.ManyToMany; import java.util.Set; @Entity @Data public class Student { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String name; @ManyToMany(mappedBy = "students") private Set<Course> courses; } // Course.java import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.ManyToMany; import java.util.Set; @Entity @Data public class Course { @Id @GeneratedValue(stratergy = GenerationType.Identity) private Long id; private String title; @ManyToMany private Set<Student> students; }
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