JPA Query and Relations

Question

I'm developing an HRM application so I'm confused to how manage entities over JPA.

My case resuming in is a set of tables in a multilingual context: -employees -departments -languages -departments_languages

following my database tables:

Employee Entity class:

  @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
  }

Department Entity class

@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;

    ...
}

DepartmentLanguage Entity

        @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;

            ...
        }

Firtly, I ask you if is a correct way and how can I perform JPA queries as:

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

From this query I need Employee informations, with department name (assuming that languageId is 1 )

Performing a query from eclipse JPA console return an Employee object with all relationship, but both all correlated results

Can you show me a sample query for this kind of design please

Answer 1

Solved realizing query using fetch join, for example:

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");

Also you can use EntiyGraph feature to both improve performance in a great JPA style ;)