Spring/JPA/Hibernate How to Perform Join of Two Entities In One Repository

Question

First let's say we have two tables. One table is an Employee table with the following columns:

EMPLOYEE:
------------------------
emp_id (int, primary key)
emp_name (varchar(125))
emp_dept (foreign key)
emp_intro (text)

The other table is a Department table with the following columns:

DEPARTMENT:
-----------
dept_id (int, primary key)
dept_label (varchar(25))

Here is a sample of the table's values

DEPARTMENT:
------------------------
dept_id    | dept_label
------------------------
 1         | Sales
------------------------
 2         | Technology
------------------------
 3         | Finance

In order to return the employee's info with a status label, we need to either perform a JOIN:

SELECT e, d.dept_label FROM employees JOIN department d ON d.dept_id = e.emp_dept

or a multi-table select:

SELECT e.emp_id, e.emp_name, d.dept_label, e.emp_intro FROM employees e, department d WHERE e.emp_dept = d.dept_id

However, when using JPA/Hibernate, we need to create two classes:

Employee.java

package com.example.entities;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "employees")
public class Employee implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "emp_id")
    private long emp_id;

    @Column(name = "emp_name")
    private String emp_name;

    @Column(name = "emp_dept")
    private Integer emp_dept;

    @Column(name = "emp_intro")
    private String emp_intro;

    public long getEmp_id() {
        return emp_id;
    }

    public void setEmp_id(long emp_id) {
        this.emp_id = emp_id;
    }

    public String getEmp_name() {
        return emp_name;
    }

    public void setEmp_name(String emp_name) {
        this.emp_name = emp_name;
    }

    public Integer getEmp_dept() {
        return emp_dept;
    }

    public void setEmp_dept(Integer emp_dept) {
        this.emp_dept = emp_dept;
    }

    public String getEmp_intro() {
        return emp_intro;
    }

    public void setEmp_intro(String emp_intro) {
        this.emp_intro = emp_intro;
    }

    public static long getSerialversionuid() {
        return serialVersionUID;
    }

}

Department.java

package com.example.entities;

import java.io.Serializable;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "departments")
public class Department implements Serializable {
    private static final long serialVersionUID = 1L;

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "dept_id")
    private long dept_id;

    @Column(name = "dept_label")
    private String dept_label;

    public long getDept_id() {
        return dept_id;
    }

    public void setDept_id(long dept_id) {
        this.dept_id = dept_id;
    }

    public String getDept_label() {
        return dept_label;
    }

    public void setDept_label(String dept_label) {
        this.dept_label = dept_label;
    }

    public static long getSerialversionuid() {
        return serialVersionUID;
    }

}

Then, there is the repository (DAO):

EmployeeRepository

package com.example.repository;

import.java.util.List;

import org.springframework.data.jpa.repository.JpaRepository;
import org.springframework.data.jpa.repository.Query;

import com.example.entities.Employee;

public interface EmployeeRepository extends JpaRepository<Employee, Long> {

    @Query("select e, d.dept_label FROM Employee e JOIN Department d ON "
    + "d.dept_id = e.emp_id")
    public List<Employee> return getEmployees();

}

and lastly, the Java controller that binds the classed query to an endpoint of the application:

EmployeeController.java

package com.example.controllers;

import java.util.List;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;

import com.example.entities.Department;
import com.example.entities.Employee;
import com.example.repository.EmployeeRepository;

@Controller
public class EmployeeController {

    @Autowired
    EmployeeRepository er;

    @RequestMapping(value = "/getEmployees")
    public @ResponseBody List<Employee> getEmployees() {
        return er.getEmployees();
    }

}

I have already tested this entire structure with only retrieving rows inside of the Employee table (i.e. @Query("SELECT e FROM Employee e") ) and everything returns as is.

MY MAIN ISSUE is how does one return a JOIN QUERY while the query is inside of a specific class (table), being Employee, if I require contents inside of Department?

I've already tried @JoinColumn annotations and that didn't work as well (perhaps I did it wrong).

Any ideas? Thanks.

Answer 1

You dont have to use raw joins to do that, just use proper relation mapping. Relation between Employee and Departament sounds like @ManyToOne or @ManyToMany. You will be able to eg employee.getDepartament() or query by employee.departament.name=:name

http://www.objectdb.com/api/java/jpa/ManyToMany

You can even map bidirectional relations so you will be able to get deparament from employee, as well as all employees from given deparaments

PS. @JoinColumn is used to delare DB columnt used for joins it it is different then created by selected named strategies (usualy entityname_id). Actual relation mapping is done by declaring @OneToOne @OneToMany @ManyToMany and those can but doesn't have to be used with @JoinColumn. It is strict JPA question.

Here you have complete documentation of JPA 2.1 specification It describes in details how to declare relations as well as @MappedSuperclass, inheritance strategies and all other usefull stuff.