Hibernate之表間關系

ManyToOne

? ? ? ? ?多對一，是最常見的表間關系，對應關系數據庫中的外鍵關系。通常用于建立子實體和其父實體的關聯關系

@Entity(name = "Person")
public static class Person {@Id@GeneratedValueprivate Long id;//Getters and setters are omitted for brevity

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

? ? ? ? 一對多用于建立父實體和子實體之間的關系。如果子實體側沒有對應的ManyToOne配置，則這個OneToMany是單向的。如果子實體側有對應的ManyToOne配置，則這個OneToMany是雙向的。雙向的關系，可以讓開發者在兩側都能獲取關聯關系。

? ? ?單向的一對多關系，實例：

@Entity(name = "Person")
public static class Person {@Id@GeneratedValueprivate 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@GeneratedValueprivate 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 PhoneALTER TABLE Person_Phone
ADD CONSTRAINT FK2ex4e4p7w1cj310kg2woisjl2
FOREIGN KEY (Person_id) REFERENCES Person

? ? ?Hibernate對單向的一對多關系，兩個實體對應兩個表，關聯關系使用一個中間表來表達。單向一對多在級聯操作上比較低效。

?

雙向一對多

? ? Hibernate處理雙向一對多關系，按多對一的關系來處理，本質上還是主外鍵關系。雙向的一對多使父子雙方都能能力來獲取關聯關系。使操作更方便，效率等同多對一。

? ??

@Entity(name = "Person")
public static class Person {@Id@GeneratedValueprivate Long id;@OneToMany(mappedBy = "person", cascade = CascadeType.ALL, orphanRemoval = true)private List<Phone> phones = new ArrayList<>();//Getters and setters are omitted for brevitypublic 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@GeneratedValueprivate Long id;@NaturalId@Column(name = "`number`", unique = true)private String number;@ManyToOneprivate Person person;//Getters and setters are omitted for brevity
@Overridepublic 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 );}@Overridepublic 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

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