jpa之一对多关系

jpa之一对多关系

单向一对多

单向一对多只需要在一方加@OneToMany,而多方则什么都不用添加

DepartMentDo.java:

@Entity
@Table(name = "t_depart")
public class DepartMentDo {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String name;

    @OneToMany(targetEntity = EmployeeDo.class, fetch = FetchType.LAZY)
    private Collection employees;
    //getter and setter
}

@Entity
@Table(name = "t_emp")
public class EmployeeDo {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String name;
}

生成表t_depart,t_emp和关系维护表t_depart_employees,关系维护表由``一方表名[email protected]`注解上属性名称组成

mysql> desc t_depart;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int(11)      | NO   | PRI | NULL    | auto_increment |
| name  | varchar(255) | YES  |     | NULL    |                |
+-------+--------------+------+-----+---------+----------------+
2 rows in set

mysql> desc t_emp;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int(11)      | NO   | PRI | NULL    | auto_increment |
| name  | varchar(255) | YES  |     | NULL    |                |
+-------+--------------+------+-----+---------+----------------+
2 rows in set

mysql> desc t_depart_employees;
+-------------------+---------+------+-----+---------+-------+
| Field             | Type    | Null | Key | Default | Extra |
+-------------------+---------+------+-----+---------+-------+
| depart_ment_do_id | int(11) | NO   | MUL | NULL    |       |
| employees_id      | int(11) | NO   | PRI | NULL    |       |
+-------------------+---------+------+-----+---------+-------+
2 rows in set

双向一对多

多的一方就是关系的拥有者即有@JoinColumn的一方,mappedBy指的是关系的反方,值的话一般是关系拥有者中的属性(field)，即EmployeeDo实体中的departMentDo属性字段对应的值

DepartMentDo.java:

@Entity
@Table(name = "t_depart")
public class DepartMentDo {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String name;

    @OneToMany(targetEntity = EmployeeDo.class, fetch = FetchType.LAZY, mappedBy = "departMentDo")
    private Collection employee;

    //getter and setter
}

EmployeeDo.java:

@Entity
@Table(name = "t_emp")
public class EmployeeDo {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private int id;
    private String name;

    @ManyToOne
    @JoinColumn(name = "dept_id")
    private DepartMentDo departMentDo;

    //getter and setter
}

生成的表:

mysql> desc t_depart;
+-------+--------------+------+-----+---------+----------------+
| Field | Type         | Null | Key | Default | Extra          |
+-------+--------------+------+-----+---------+----------------+
| id    | int(11)      | NO   | PRI | NULL    | auto_increment |
| name  | varchar(255) | YES  |     | NULL    |                |
+-------+--------------+------+-----+---------+----------------+
2 rows in set

mysql> desc t_emp;
+---------+--------------+------+-----+---------+----------------+
| Field   | Type         | Null | Key | Default | Extra          |
+---------+--------------+------+-----+---------+----------------+
| id      | int(11)      | NO   | PRI | NULL    | auto_increment |
| name    | varchar(255) | YES  |     | NULL    |                |
| dept_id | int(11)      | YES  | MUL | NULL    |                |
+---------+--------------+------+-----+---------+----------------+