JPA关系映射

时间 2019-12-08

标签 jpa 关系映射栏目 Java 繁體版

原文原文链接

JPA关系映射

四种映射关系

在关系数据库中, 有表与表存在下面几种关系java

多对一关系(Many-to-One)
一对一关系(One-to-One)
一对多关系(One-to-Many)
多对多关系(Many-to-Many)

也对应这JPA中四种映射关系.sql

单值串联

从一个实体实例关联到另外一个实体实例上, 其中目标基数为1, 成为单值关联(single-valued association).
多对一关系和一对一关系都是属于这一类, 由于源实体至多引用了一个目标实体.数据库

多对一关联(ManyToOne)

来看这么一个关系, 员工(employee)和部门(department)之间的关系, 显然一个员工只能属于一个部门, 而一个部门
内固然拥有多个员工, 因此从员工到部门的映射关系是多对一关系.app

employee实体类ide

@Entity
public class Employee {
    @Id
    private Integer id;
    private String name;
    private Long salary;

    @ManyToOne
    private Department department;

    public Employee() {
    }

    public Employee(Integer id) {
        this.id = id;
    }
}

department实体类this

@Data
@Entity
public class Department {
    @Id
    private Integer id;

    private String name;

}

程序运行后, jpa建立表的sql语句以下spa

+------------+--------------------------------------+
| Table      | Create Table                         |
+------------+--------------------------------------+
| department | CREATE TABLE `department` (          |
|            |   `id` int(11) NOT NULL,             |
|            |   `name` varchar(255) DEFAULT NULL,  |
|            |   PRIMARY KEY (`id`)                 |
|            | ) ENGINE=InnoDB DEFAULT CHARSET=utf8 |
+------------+--------------------------------------+

+----------+---------------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                            |
+----------+---------------------------------------------------------------------------------------------------------+
| employee | CREATE TABLE `employee` (                                                                               |
|          |   `id` int(11) NOT NULL,                                                                                |
|          |   `name` varchar(255) DEFAULT NULL,                                                                     |
|          |   `salary` bigint(20) DEFAULT NULL,                                                                     |
|          |   `department_id` int(11) DEFAULT NULL,                                                                 |
|          |   PRIMARY KEY (`id`),                                                                                   |
|          |   KEY `FKbejtwvg9bxus2mffsm3swj3u9` (`department_id`),                                                  |
|          |   CONSTRAINT `FKbejtwvg9bxus2mffsm3swj3u9` FOREIGN KEY (`department_id`) REFERENCES `department` (`id`) |
|          | ) ENGINE=InnoDB DEFAULT CHARSET=utf8                                                                    |
+----------+---------------------------------------------------------------------------------------------------------+

department_id是jpa建立的外键, 若是想指定外键的名字, 能够经过@JoinColumn来声明外键的名字hibernate

一对一映射

对于员工来讲, 只有一个停车位(PackingSpace), 因此员工对停车位来讲是一对一映射关系
修改EMPLOYEE类code

@Data
@Entity
public class Employee {
    @Id
    private Integer id;
    private String name;
    private Long salary;

    @ManyToOne
    @JoinColumn(name = "dept_id")
    private Department department;

    @OneToOne
    @JoinColumn(name = "p_space_id")
    private ParkingSpace parkingSpace;

    public Employee() {
    }

    public Employee(Integer id) {
        this.id = id;
    }
}

建立PackingSpace实体类ci

@Data
@Entity
public class ParkingSpace {
    @Id
    private Integer id;

    private String location;
}

查看parking_space表和employee表

+---------------+-----------------------------------------+
| Table         | Create Table                            |
+---------------+-----------------------------------------+
| parking_space | CREATE TABLE `parking_space` (          |
|               |   `id` int(11) NOT NULL,                |
|               |   `location` varchar(255) DEFAULT NULL, |
|               |   PRIMARY KEY (`id`)                    |
|               | ) ENGINE=InnoDB DEFAULT CHARSET=utf8    |
+---------------+-----------------------------------------+
+----------+---------------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                            |
+----------+---------------------------------------------------------------------------------------------------------+
| employee | CREATE TABLE `employee` (                                                                               |
|          |   `id` int(11) NOT NULL,                                                                                |
|          |   `name` varchar(255) DEFAULT NULL,                                                                     |
|          |   `salary` bigint(20) DEFAULT NULL,                                                                     |
|          |   `dept_id` int(11) DEFAULT NULL,                                                                       |
|          |   `p_space_id` int(11) DEFAULT NULL,                                                                    |
|          |   PRIMARY KEY (`id`),                                                                                   |
|          |   KEY `FKaqchbcb8i6nvtl9g6c72yba0p` (`dept_id`),                                                        |
|          |   KEY `FKd383146ko181lfhm1xuy3arci` (`p_space_id`),                                                     |
|          |   CONSTRAINT `FKaqchbcb8i6nvtl9g6c72yba0p` FOREIGN KEY (`dept_id`) REFERENCES `department` (`id`),      |
|          |   CONSTRAINT `FKd383146ko181lfhm1xuy3arci` FOREIGN KEY (`p_space_id`) REFERENCES `parking_space` (`id`) |
|          | ) ENGINE=InnoDB DEFAULT CHARSET=utf8                                                                    |
+----------+---------------------------------------------------------------------------------------------------------+

双向一对一映射

对于员工来讲, 至多只能拥有一个停车位, 对于停车位来讲, 也至多只能属于一个员工, 因此两个方向都是一对一关系, 这被称为双向一对一关系

修改ParkingSpace实体类

@Data
@Entity
public class ParkingSpace {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    private String location;

    @OneToOne(mappedBy = "parkingSpace")
    @JoinColumn(name = "emp_id")
    private Employee employee;

    @Override
    public String toString() {
        return "ParkingSpace{" +
                "id=" + id +
                ", location='" + location + '\'' +
                '}';
    }
}

在双向一对一关系中, 若是双方都有mapperBy, name这样是不合法, 若是双方都没有mapperBy, 这样是不正确的, 由于若是双方都没有mapperBy,
则当前不是双向一对一关系, 而是两个单向的一对一关系.

集合值关联

当源实体引用一个或者多个目标实体实例时, 将使用一个多值关联(many-valued association). 一对多关联和多对多映射都符合

一对多映射

前面对于员工来讲, 对于部门是多对一映射, 多个员工属于一个部门, 相反的, 对于部门来讲, 对员工是一对多的映射, 对于一个部门来讲, 拥有多个员工,
因此也是双向的关系

修改department实体类

@Data
@Entity
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    private String name;

   @OneToMany(mappedBy = "department")
    private List<Employee> employees;
}

多对多映射

员工与项目的关系也是多对多的关系, 一个员工大几率不会只会作一个项目, 一个项目大几率也不会只交给一个员工作

增长项目实体类

@Data
@Entity
public class Project {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    private String name;

    @ManyToMany(mappedBy = "projects")
    private List<Employee> employees;
}

在员工实体类中添加以下

@ManyToMany
    private List<Project> projects;

查看建立的表

+---------------------+
| Tables_in_db_projpa |
+---------------------+
| department          |
| employee            |
| employee_projects   |
| hibernate_sequence  |
| parking_space       |
| project             |
+---------------------+

能够看到除了建立project表外, 还建立了表employee_projects表, 没有对这个链接表进行配置, 是根据jpa默认规则进行建立的, 创表的sql语句为

+-------------------+-------------------------------------------------------------------------------------------------------+
| Table             | Create Table                                                                                          |
+-------------------+-------------------------------------------------------------------------------------------------------+
| employee_projects | CREATE TABLE `employee_projects` (                                                                    |
|                   |   `employees_id` int(11) NOT NULL,                                                                    |
|                   |   `projects_id` int(11) NOT NULL,                                                                     |
|                   |   KEY `FKg10a7uho2lylw8g080l5j4gyk` (`projects_id`),                                                  |
|                   |   KEY `FK88mdj5vmp7md19u4cbq154dk6` (`employees_id`),                                                 |
|                   |   CONSTRAINT `FK88mdj5vmp7md19u4cbq154dk6` FOREIGN KEY (`employees_id`) REFERENCES `employee` (`id`), |
|                   |   CONSTRAINT `FKg10a7uho2lylw8g080l5j4gyk` FOREIGN KEY (`projects_id`) REFERENCES `project` (`id`)    |
|                   | ) ENGINE=InnoDB DEFAULT CHARSET=utf8                                                                  |
+-------------------+-------------------------------------------------------------------------------------------------------+

使用@JoinTable来配置链接表

修改员工实体类, 添加@JoinTable注解

@ManyToMany
    @JoinTable(name = "emp_proj", joinColumns = @JoinColumn(name = "emp_id"), inverseJoinColumns = @JoinColumn(name = "proj_id"))
    private List<Project> projects;

能够看到表名和字段名都相应变化了

+----------+-------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                    |
+----------+-------------------------------------------------------------------------------------------------+
| emp_proj | CREATE TABLE `emp_proj` (                                                                       |
|          |   `emp_id` int(11) NOT NULL,                                                                    |
|          |   `proj_id` int(11) NOT NULL,                                                                   |
|          |   KEY `FKj350n5wr8yyesit27cw0agpjq` (`proj_id`),                                                |
|          |   KEY `FK10tf71i5h7os8kdrpm2k97gn2` (`emp_id`),                                                 |
|          |   CONSTRAINT `FK10tf71i5h7os8kdrpm2k97gn2` FOREIGN KEY (`emp_id`) REFERENCES `employee` (`id`), |
|          |   CONSTRAINT `FKj350n5wr8yyesit27cw0agpjq` FOREIGN KEY (`proj_id`) REFERENCES `project` (`id`)  |
|          | ) ENGINE=InnoDB DEFAULT CHARSET=utf8                                                            |
+----------+-------------------------------------------------------------------------------------------------+

参考资料

Pro JPA 2: 精通Java持久化API