一對多的關係

一對多對映通常只是多對一對映的雙向關係。我們將採用與多對一對映相同的示例。

Employee.java

@Entity
public class Employee {

    @TableGenerator(name = "employee_gen", table = "id_gen", pkColumnName = "gen_name", valueColumnName = "gen_val", allocationSize = 100)
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "employee_gen")
    private int idemployee;
    private String firstname;
    private String lastname;
    private String email;

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

    // getters and setters    
}

Department.java

@Entity
public class Department {

    @TableGenerator(table = "id_gen", pkColumnName = "gen_name", valueColumnName = "gen_val", name = "department_gen", allocationSize = 1)
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, generator = "department_gen")
    private int iddepartment;
    private String name;

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

    // getters and setters    
}

這種關係在資料庫中表示如下。

關於 jpa 一對多對映有兩點需要記住：

• 多對一方是關係的擁有方。該列定義在該側。
• 一對多對映是反面，因此必須在反面使用 mappedBy 元素。

完整的例子可以在這裡提到