Hibernate one-to-many to fetch only primary key column of the target entity-CodePudding

I have following two JPA entities : Company and Employee which follow one-to-many relationship :

Employee
---------
eId  |  eName   | company_id

Company
--------
cId  |  cName

I have been trying to fetch Employee IDs list in Company entity. I tried with two ways - employees1 and employees2 as shown below. Both are not working. Note that I want List of Employee IDs and not the List of Employees.

@Entity
public class Employee {
   @Id
   String eId;
   String eName;
}

@Entity
public class Company{
   @Id
   String cId;
   String cName;

   // both of the below are not working
   
   @OneToMany(fetch = EAGER, targtEntity = Employee.class )
   @JoinColumn(name = "company_id", referencedColumn = "cId")
   @Column(name = "eId")
   Set<String> employees1 = new HashSet<>();
   

   @OneToMany(fetch = EAGER)
   @JoinColumn(name = "company_id", tableName = "Employee", referencedColumn = "cId")
   @Column(name = "eId")
   Set<String> employees2 = new HashSet<>();
}

For employees1, I am getting : org.hibernate.property.access.spi.PropertyAccessException: Error accessing field.

For employees2 : org.hibernate.cfg.NotYetImplementedException: Collections having FK in secondary table.

Please help!

CodePudding user response：

Check out this article by Vlad Mihalcea, it explains everything in great detail! The optimal mapping would look like this:

@Entity
public class Employee {

    @Id
    private String eId;

    private String eName;

    @ManyToOne(fetch = FetchType.LAZY)
    private Company refundRequest;

    void setRefundRequest(Company refundRequest) {
        this.refundRequest = refundRequest;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        Employee employee = (Employee) o;
        return Objects.equals(eId, employee.eId);
    }

    @Override
    public int hashCode() {
        return Objects.hash(eId);
    }
    
}

@Entity
public class Company {

    @Id
    private String cId;

    private String cName;

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "company", orphanRemoval = true)
    private List<Employee> employees;

    public void addEmployee(final Employee employee) {
        employee.setRefundRequest(this);
        employees.add(employee);
    }
    
    public void removeEmployee(final Employee employee) {
        employee.setRefundRequest(null);
        employees.remove(employee);
    }
    
}

Make sure to read the above mentioned article to understand why this mapping is optimal!

In order to fetch it, you can use a repository:

@Repository
public interface EmployeeRepository extends JpaRepository<Employee, String> {

    @Query("select e.eId from Employee e where e.company = :company")
    List<String> findEmployeeIdsByCompany(@Param("company") Company company);

}

By the way, eager fetching is a massive code smell and should be avoided!

CodePudding user response：

@ElementCollection is the annotation you need to use in this case. @CollectionTable defines in which table you want to fetch the value from as well as on which column the join happens. @Column defines the column name you want to fetch from the @CollectionTable

@Entity
class Company {
    @Id
    String cId;
    String cName;

    @ElementCollection
    @CollectionTable(name = "Employee", joinColumns = @JoinColumn(name = "company_id"))
    @Column(name = "eId")
    Set<String> employeeIds = new HashSet();

}