JPA查询和关系

Question

我正在开发一个人力资源管理应用程序，所以我对如何通过JPA管理实体感到困惑。

我的情况是一组多语言上下文中的表:-employees-departments-languages-departments_languages

在我的数据库表之后：

  @Entity
  @Table(name="employees")
  @NamedQuery(name="Employee.findAll", query="SELECT e FROM Employee e")
  public class Employee implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Column(name="employeed_id", unique=true, nullable=false)
    private int employeedId;

    @Column(nullable=false, length=255)
    private String address;

    @Temporal(TemporalType.DATE)
    @Column(nullable=false)
    private Date birthday;

    ...

    //bi-directional many-to-one association to Department
    @ManyToOne(fetch=FetchType.LAZY)
    @JoinColumn(name="department_id", nullable=false)
    private Department department
  }

@Entity
@Table(name="departments")
@NamedQuery(name="Department.findAll", query="SELECT d FROM Department d")
public class Department implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    @Column(name="department_id", unique=true, nullable=false)
    private int departmentId;

    @Temporal(TemporalType.TIMESTAMP)
    @Column(name="date_added", nullable=false)
    private Date dateAdded;

    @Column(name="location_id", nullable=false)
    private int locationId;

    @Column(nullable=false)
    private byte status;

    //bi-directional many-to-one association to DepartmentsLanguage
    @OneToMany(mappedBy="department")
    private Set<DepartmentsLanguage> departmentsLanguages;

    //bi-directional many-to-one association to Employee
    @OneToMany(mappedBy="department",fetch=FetchType.LAZY)
    private Set<Employee> employees;

    ...
}

        @Entity
        @Table(name="departments_languages")
        @NamedQuery(name="DepartmentsLanguage.findAll", query="SELECT d FROM DepartmentsLanguage d")
        public class DepartmentsLanguage implements Serializable {
            private static final long serialVersionUID = 1L;

            @EmbeddedId
            private DepartmentsLanguagePK id;

            @Column(length=255)
            private String description;

            @Column(length=255)
            private String name;

            //bi-directional many-to-one association to Department
            @ManyToOne(fetch=FetchType.LAZY)
            @JoinColumn(name="department_id", nullable=false, insertable=false, updatable=false)
            private Department department;

            //bi-directional many-to-one association to Language
            @ManyToOne(fetch=FetchType.LAZY)
            @JoinColumn(name="language_id", nullable=false, insertable=false, updatable=false)
            private Language language;

            ...
        }

SELECT * FROM employees
INNER JOIN departments_languages using(department_id)
WHERE employeed_id = 1 and language_id = 1

从这个查询中，我需要员工信息，以及部门名称（假设languageId为1）

从eclipse JPA控制台执行查询将返回一个Employee对象，该对象具有所有关系，但结果都是相关的

你能给我看一个这种设计的样本查询吗

Answer 1

解决了使用fetch join实现查询的问题，例如：

Query query = entityManager.createQuery("select e from Employee as e "
    + "JOIN FETCH e.department d "
    + "JOIN FETCH d.departmentsLanguages dl "
    + "where e.employeedId = :employeeId "
    + "and dl.language = :languageId");

此外，您还可以使用EntiyGraph特性以一种出色的JPA风格来提高性能；）