jpa无外键配置

在用jpa这种orm框架时，有时我们实体对象存在关联关系，但实际的业务场景可能不需要用jpa来控制数据库创建数据表之间的关联约束，这时我们就需要消除掉数据库表与表之间的外键关联。
但jpa在处理建立外键时存在一些问题，在stackoverflow上搜索了相关的jpa创建实体对象关联关系但不建立外键这一系列问题后，发现这个是jpa在处理外键时存在一定的bug，官方给出的答复是在hibernate 5.x会解决掉这个问题，但是经验证5.x的版本这个问题依旧存在。下面给出这个问题的解释以及这个问题如何解决。

下面会以techer和student对象来举例，teacher和student存在一对多关系，一个teacher关联多个student。

1.teacher与student设置外键关系

teacher和student之间通过@OneToMany和@ManyToOne建立外键关联关系
teacher:

@Entity
@Table(name = "TEACHER")
public class Teacher extends BaseDomain {
    @Id()
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID")
    private Long id;

    @Column
    private String name;

    @OneToMany(mappedBy = "teacher")
    private List<Student> students;

    //getter&setter...
}

student:

@Entity
@Table(name = "STUDENT")
public class Student extends BaseDomain {

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

    @Column
    private String name;

    @ManyToOne
    @JoinColumn(name = "tid")
    private Teacher teacher;

    //getter&setter...
}

数据库生成表结果:

CREATE TABLE “public”.”student” (
“id” int8 DEFAULT nextval(‘student_id_seq’::regclass) NOT NULL,
“name” varchar(255) COLLATE “default”,
“teacher_id” int8,
CONSTRAINT “student_pkey” PRIMARY KEY (“id”),
CONSTRAINT “fk3y5qg5r9ewc48x7ek8lx5ua8h” FOREIGN KEY (“teacher_id”) REFERENCES “public”.”teacher” > (“id”) ON DELETE NO ACTION ON UPDATE NO ACTION
)
WITH (OIDS=FALSE)
;
ALTER TABLE “public”.”student” OWNER TO “postgres”;

可以看到设置了外键”fk3y5qg5r9ewc48x7ek8lx5ua8h” FOREIGN KEY (“teacher_id”)

2.只在student端加上@ForeignKey

student

@Entity
@Table(name = "STUDENT")
public class Student extends BaseDomain {

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

    @Column
    private String name;

    @ManyToOne
    @JoinColumn(name = "tid",foreignKey = @ForeignKey(name = "none",value = ConstraintMode.NO_CONSTRAINT))
    private Teacher teacher;

    //setter&getter
}

加上该注解之后，根据这个注解说明是可以去掉外键关联关系，但发现加上后然并卵,外键还是没有去掉。这里需要说明其中@ForeignKey的value值由如下代码所示几种情况:

/**
 * Used to control the application of a constraint.
 *
 * @since JPA 2.1
 */
public enum ConstraintMode {
    /**
     * Apply the constraint.
     */
    CONSTRAINT,
    /**
     * Do not apply the constraint.
     */
    NO_CONSTRAINT,
    /**
     * Use the provider-defined default behavior.
     */
    PROVIDER_DEFAULT
}

3.在teacher端加入@org.hibernate.annotations.ForeignKey(name = “none”)

在一的这端加上@org.hibernate.annotations.ForeignKey(name = “none”)这个被jpa废弃的注解。加上之前在student中设置的@ForeignKey（注意这个是javax.persistence包下的），可以去掉外键关联
teacher:

@Entity
@Table(name = "TEACHER")
public class Teacher extends BaseDomain {
    @Id()
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "ID")
    private Long id;

    @Column
    private String name;

    @OneToMany(mappedBy = "teacher")
    @org.hibernate.annotations.ForeignKey(name = "none")
    private List<Student> students;

    //getter&setter...
}

结果:

CREATE TABLE “public”.”student” (
“id” int8 DEFAULT nextval(‘student_id_seq’::regclass) NOT NULL,
“createdtime” timestamp(6),
“name” varchar(255) COLLATE “default”,
“version” int4,
“tid” int8,
CONSTRAINT “student_pkey” PRIMARY KEY (“id”)
)
WITH (OIDS=FALSE)
;
ALTER TABLE “public”.”student” OWNER TO “postgres”;

可以看到student表中原来关联teacher的外键没了，说明该注解起作用。

4.需要注意的坑

1.如果teacher（1的这端）有student列表(多的这端)，像这样：

@OneToMany(mappedBy = "teacher")
@org.hibernate.annotations.ForeignKey(name = "none")
private List<Student> students;

如果要去掉外键关联关系，student端也需要像在2小结提到样需要加上@JoinColumn(name = “tid”,foreignKey = @ForeignKey(name = “none”,value = ConstraintMode.NO_CONSTRAINT)),但此时你会发现其中@ForeignKey中value的值不管你设置为ConstraintMode.NO_CONSTRAINT还是ConstraintMode.CONSTRAINT，数据库都不会设置外键(这个是特么的真奇怪)。但是就是不管怎样，你就是不能不设置@ForeignKey，并且你还必须要设置其中的值不能为默认值，不然就是要生成外键。这里贴下@ForeignKey的源码:

@Target({})
@Retention(RUNTIME)
public @interface ForeignKey {
    /**
     * (Optional) The name of the foreign key constraint.  Defaults to a provider-generated name.
     *
     * @return The foreign key name
     */
    String name() default "";

    /**
     * (Optional) The foreign key constraint definition.  Default is provider defined.  If the value of
     * disableForeignKey is true, the provider must not generate a foreign key constraint.
     *
     * @return The foreign key definition
     */
    String foreignKeyDefinition() default "";

    /**
     * (Optional) Used to specify whether a foreign key constraint should be generated when schema generation is in effect.
     */
    ConstraintMode value() default ConstraintMode.CONSTRAINT;
}

真的是X了狗了。。。我表示久久不能理解。。。

2.teacher(1这端)没有student列表或者student列表被@Transient所修饰，像这样:

@OneToMany(mappedBy = "teacher")
@Transient
private List<Student> students;

那么也是无论你在student端设置ConstraintMode的值，都不会设置外键.but!!!你就是不能不在student端(多的这端)设置@JoinColumn(name=”tid”,foreignKey=@ForeignKey(name=”none”,value=ConstraintMode.NO_CONSTRAINT)),否则也是会生成外键

总结

所以要使数据表中没有外键关联关系。
1.当两边都有关联关系字段，1的这端利用@org.hibernate.annotations.ForeignKey(name = “none”)，多的那端在JoinColumn中加上foreignKey = @ForeignKey(name = “none”,value = ConstraintMode.NO_CONSTRAINT)

2.当只有多的那端有关联字段，一的那段没有关联字段或者关联字段被@Transient所修饰，请在多的那端在JoinColumn中加上foreignKey = @ForeignKey(name = “none”,value = ConstraintMode.NO_CONSTRAINT)

最后需要说明的是@org.hibernate.annotations.ForeignKey(name = “none”)这个注解之后可能会在之后的版本会被直接移除掉，所以更新jar包的时候需要注意下。

参考资料:
①.https://hibernate.atlassian.net/browse/HHH-8805
②.https://hibernate.atlassian.net/browse/HHH-8862