@ManyToOne JPA Example

The @ManyToOne annotation is used to specify a many-to-one database relationship.

@ManyToOne JPA Annotation Example

@ManyToOne is the most common association, having a direct equivalent in the relational database as well (e.g. foreign key), and so it establishes a relationship between a child entity and a parent.

Person.java

@Entity(name = "Person")
public static class Person {

	@Id
	@GeneratedValue
	private Long id;

	//Getters and setters are omitted for brevity

}

Phone.java

@Entity(name = "Phone")
public static class Phone {

	@Id
	@GeneratedValue
	private Long id;

	@Column(name = "`number`")
	private String number;

	@ManyToOne
	@JoinColumn(name = "person_id",
			foreignKey = @ForeignKey(name = "PERSON_ID_FK")
	)
	private Person person;

	//Getters and setters are omitted for brevity

}

Here is the SQL script for DB tables:

CREATE TABLE Person (
    id BIGINT NOT NULL ,
    PRIMARY KEY ( id )
)

CREATE TABLE Phone (
    id BIGINT NOT NULL ,
    number VARCHAR(255) ,
    person_id BIGINT ,
    PRIMARY KEY ( id )
 )

ALTER TABLE Phone
ADD CONSTRAINT PERSON_ID_FK
FOREIGN KEY (person_id) REFERENCES Person

Test

Person person = new Person();
entityManager.persist( person );

Phone phone = new Phone( "123-456-7890" );
phone.setPerson( person );
entityManager.persist( phone );

entityManager.flush();
phone.setPerson( null );

The above code produces below SQL statements:

INSERT INTO Person ( id )
VALUES ( 1 )

INSERT INTO Phone ( number, person_id, id )
VALUES ( '123-456-7890', 1, 2 )

UPDATE Phone
SET    number = '123-456-7890',
       person_id = NULL
WHERE  id = 2