Hibernate之表间关系

ManyToOne

         多对一，是最常见的表间关系，对应关系数据库中的外键关系。通常用于建立子实体和其父实体的关联关系

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

    @Id
    @GeneratedValue
    private Long id;

    //Getters and setters are omitted for brevity

}

@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

}

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

例子：

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 );
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

OneToMany

        一对多用于建立父实体和子实体之间的关系。如果子实体侧没有对应的ManyToOne配置，则这个OneToMany是单向的。如果子实体侧有对应的ManyToOne配置，则这个OneToMany是双向的。双向的关系，可以让开发者在两侧都能获取关联关系。

     单向的一对多关系，实例：

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

    @Id
    @GeneratedValue
    private Long id;

    @OneToMany(cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Phone> phones = new ArrayList<>();

    //Getters and setters are omitted for brevity

}

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

    @Id
    @GeneratedValue
    private Long id;

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

    //Getters and setters are omitted for brevity

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

CREATE TABLE Person_Phone (
    Person_id BIGINT NOT NULL ,
    phones_id BIGINT NOT NULL
)

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

ALTER TABLE Person_Phone
ADD CONSTRAINT UK_9uhc5itwc9h5gcng944pcaslf
UNIQUE (phones_id)

ALTER TABLE Person_Phone
ADD CONSTRAINT FKr38us2n8g5p9rj0b494sd3391
FOREIGN KEY (phones_id) REFERENCES Phone

ALTER TABLE Person_Phone
ADD CONSTRAINT FK2ex4e4p7w1cj310kg2woisjl2
FOREIGN KEY (Person_id) REFERENCES Person

     Hibernate对单向的一对多关系，两个实体对应两个表，关联关系使用一个中间表来表达。单向一对多在级联操作上比较低效。

双向一对多

    Hibernate处理双向一对多关系，按多对一的关系来处理，本质上还是主外键关系。双向的一对多使父子双方都能能力来获取关联关系。使操作更方便，效率等同多对一。

    

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

    @Id
    @GeneratedValue
    private Long id;

    @OneToMany(mappedBy = "person", cascade = CascadeType.ALL, orphanRemoval = true)
    private List<Phone> phones = new ArrayList<>();

    //Getters and setters are omitted for brevity

    public void addPhone(Phone phone) {
        phones.add( phone );
        phone.setPerson( this );
    }

    public void removePhone(Phone phone) {
        phones.remove( phone );
        phone.setPerson( null );
    }
}

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

    @Id
    @GeneratedValue
    private Long id;

    @NaturalId
    @Column(name = "`number`", unique = true)
    private String number;

    @ManyToOne
    private Person person;

    //Getters and setters are omitted for brevity

    @Override
    public boolean equals(Object o) {
        if ( this == o ) {
            return true;
        }
        if ( o == null || getClass() != o.getClass() ) {
            return false;
        }
        Phone phone = (Phone) o;
        return Objects.equals( number, phone.number );
    }

    @Override
    public int hashCode() {
        return Objects.hash( number );
    }
}
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 UK_l329ab0g4c1t78onljnxmbnp6
UNIQUE (number)

ALTER TABLE Phone
ADD CONSTRAINT FKmw13yfsjypiiq0i1osdkaeqpg
FOREIGN KEY (person_id) REFERENCES Person