Database Relations

JPA annotations

• parent

• child

• unidirectional

• bidirectional

• Lazy

  • ...ToMany

• Eager

  • ...ToOne

• cascade

  • CascadeType.PERSIST
  • CascadeType.MERGE
  • CascadeType.REFRESH
  • CascadeType.REMOVE
  • CascadeType.DETACH
  • CascadeType.ALL

• JPA has no default

• Parent -> Child : make sense

• Child -> Parent : not useful and not recommended

Entities:

1. User
2. Address
3. Class
4. Course

Relations:

#	Entities	Relation
1	User - Address	One-To-One
2	User - Class	One-To-Many
3	Class - User	Many-To-One
4	User - Course	Many-To-Many

---

One-To-One

import jakarta.persistence.*;

@Entity
@Table(name = "users")
public class UserEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "user_id")
    private long userId;

    private String username;

    @OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL, targetEntity = AddressEntity.class)
    @JoinColumn(name = "addresses", referencedColumnName = "address_id", nullable = true)
    private AddressEntity address;
}

import jakarta.persistence.*;

@Entity
@Table(name = "addresses")
public class AddressEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "address_id")
    private long addressId;

    private String city;
}

• @JoinColumn used as a foreign key
• name defines the name of the foreign key column
• referencedColumnName indicates the field name inside the target column

---

One-To-Many & Many-To-One

Usually, the child entity is one that owns the relationship and the parent entity contains the @OneToMany annotation.

@Entity
@Table(name = "users")
public class UserEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "user_id")
    private long userId;

    private String username;

    @ManyToOne(fetch = FetchType.LAZY, optional = true)
    @JoinColumn(name = "class_id", referencedColumnName = "class_id", nullable = true)
    private ClassEntity classEntity;
}

• child entity has JoinColumn
• child entity is the owner of this relationship

@Entity
@Table(name = "classes")
public class ClassEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "class_id")
    private long id;

    private String name;

    @OneToMany(
            mappedBy = "classEntity",
            fetch = FetchType.LAZY,
            cascade = CascadeType.PERSIST,
            targetEntity = UserEntity.class
    )
    private Set<UserEntity> users;
}

• @OneToMany has mappedBy
• mappedBy means bidirectional relationship

Many-To-Many

@Entity
@Table(name = "users")
public class UserEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "user_id")
    private long userId;

    private String username;

    @ManyToMany(fetch = FetchType.EAGER, cascade = CascadeType.PERSIST)
    @JoinTable(
            name = "users_courses",
            joinColumns = {
                    @JoinColumn(name = "user_id", referencedColumnName = "user_id")
            },
            inverseJoinColumns = {
                    @JoinColumn(name = "course_id", referencedColumnName = "course_id")
            }
    )
    private Set<CourseEntity> courses = new HashSet<>();
}

@Entity
@Table(name = "courses")
public class CourseEntity {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "course_id")
    private long id;

    private String name;

    @ManyToMany(
            mappedBy = "courses",
            fetch = FetchType.EAGER,
            cascade = CascadeType.PERSIST
    )
    private Set<UserEntity> users = new HashSet<>();
}

Usually, we mention mappedBy parameter on the owning entity