JPA关系映射

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    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

    • 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.

    单值串联

    从一个实体实例关联到另一个实体实例上, 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, 因为源实体至多引用了一个目标实体.

    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
    内当然拥有多个员工, 所以从员工到部门的映射关系是多对一关系.

    employee实体类

    @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

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

    private String name;

}

    程序运行后, jpa创建表的sql语句如下

    +------------+--------------------------------------+
| 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创建的外键, 如果想指定外键的名字, 可以通过@JoinColumn来声明外键的名字

    一对一映射

    对于员工来说, 只有一个停车位(PackingSpace), 所以员工对停车位来说是一对一映射关系
    修改EMPLOYEE类

    @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;
    }
}

    创建PackingSpace实体类

    @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

    对于员工来说, 至多只能拥有一个停车位, 对于停车位来说, 也至多只能属于一个员工, 所以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, 如果双方都有mapperBy, name这样是不合法, 如果双方都没有mapperBy, 这样是不正确的, 因为如果双方都没有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

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

    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, 一个员工大概率不会只会做一个项目, 一个项目大概率也不会只交给一个员工做

    增加项目实体类

    @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如下

        @ManyToMany
    private List<Project> projects;

    查看创建的表

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

    可以看到除了创建project表外, 还创建了表employee_projects表, 没有对这个连接表进行配置, 是根据jpa默认规则进行创建的, 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来配置连接表

    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;

    可以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

    jpaspringjava
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