Spring Data JpaRepository “JOIN FETCH” returns duplicates

I’m writing a simple Spring Data JPA application. I use MySQL database. There are two simple tables:

• Department
• Employee

Each employee works in some department (Employee.department_id).

@Entity
public class Department {
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    private Long id;

    @Basic(fetch = FetchType.LAZY)
    @OneToMany(mappedBy = "department")
    List<Employee> employees;
}

@Entity
public class Employee {
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Id
    private Long id;

    @ManyToOne
    @JoinColumn
    private Department department;
}

@Repository
public interface DepartmentRepository extends JpaRepository<Department, Long> {
    @Query("FROM Department dep JOIN FETCH dep.employees emp WHERE dep = emp.department")
    List<Department> getAll();
}

The method getAll returns a list with duplicated departments (each department is repeated as many times as there are employees in this department).

Question 1: Am I rigth that this is a feature related not to Spring Data JPA, but to to Hibernate?
Question 2: What is the best way to fix it? (I found at least two ways: 1) use Set<Department> getAll(); 2) use "SELECT DISTINCT dep" in @Query annotation)

Answer

FROM Department dep JOIN FETCH dep.employees emp expression generates native query which returns plain result Department-Employee. Every Department will be returned dep.employees.size() times. This is a JPA-provider expected behavior (Hibernate in your case).

Using distinct to get rid of duplicates seems like a good option. Set<Department> as a query result makes it impossible to get the ordered result.