Spring JPA How to Display the JsonIgnored field on the inverse side in a One-To-Many Bidirectional

Question

I have 2 entities/model. Department and Employee

Relationship is 1 Department has MANY Employee.

My understanding of how to go about implementing a Bi-directional relationship of entities is:

1. Identify the Parent and Child
2. Identify the owner of relationship (in this case, Department owns Employee). Then add mappedBy='fieldname'
3. Annotate the both entities with inverse. @OneToMany on 1st entity and @ManyToOne on the 2nd entity

Below is how I designed the app.

Model

Department

@Entity
public class Department {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String departmentName;

    @OneToMany(mappedBy = "department")
    private Set<Employee> employees;

    //getters and setters...
}

Employee

@Entity
public class Employee {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String firstName;
    private String middleName;
    private String lastName;

    @ManyToOne
    @JsonIgnore
    private Department department;
}

Now, if I GET department by id (e.g. localhost:8080/department/1) I'll get :

{
    "id": 1,
    "departmentName": "Accounting",
    "employees": [
        {
            "id": 1,
            "firstName": "John",
            "middleName": "Johnny",
            "lastName": "Doe"
        }
    ]
}

which is fine because I can see the Employee(s) who belongs to a specific Department.

However, on the employee side, if I GET employee by id (e.g. localhost:8080/employee/1) I would get a json result which only displays the employee name BUT does NOT include the Department where the Employee belongs.

{
    "id": 1,
    "firstName": "John",
    "middleName": "Johnny",
    "lastName": "Doe"
}

I had to add @JsonIgnore to department field in Employee class to avoid stackoverflow/endless nested json serialization when doing a GET department (e.g. localhost:8080/department/1)

I have 2 questions :

1. How do I also display the department information when doing a GET employee by id?
2. Do I need to create a custom query in the Repository interface using @Query instead? At least for the purpose of displaying the Department name along with Employee when I do a GET Employee by id?

Thank you.

Answer 1

You have to use DTOs in your project. DTO stand for data transfer object. As the name suggest it carries data between program layers, APIs, etc. - a read on dto. Department dto may look like this for example:

public class DepartmentDto {

    private Long id;
    private String name;

    //getters setters
}

Employee dto:

public class EmployeeDto {

    private Long id;
    private String firstName;
    private DepartmentDto department;

    //getters setters
}

Then you controller method to get employee by id may look like this:

    @GetMapping("/employee/{employeeId}")
    public EmployeeDto findEmployeeById(@PathVariable Long employeeId) {
        Employee employee = this.employeeRepository.findById(employeeId).orElseThrow(() -> new RuntimeException("Employee not found"));
        DepartmentDto departmentDto = new DepartmentDto();
        departmentDto.setId(employee.getDepartment().getId());
        departmentDto.setName(employee.getDepartment().getDepartmentName());
        EmployeeDto employeeDto = new EmployeeDto();
        employeeDto.setId(employee.getId());
        employeeDto.setFirstName(employee.getFirstName());
        employeeDto.setDepartment(departmentDto);
        return employeeDto;
    }

Like this you won't get stackoverflow. Also mapping entity into a dto normally is not done by hand as in this example, but by third party libraries. You can read for some of them here - java mapping libraries.

Sometimes you may get stuck with circular references in DTO's as well. You can handle them using @JsonIdentityInfo for example. In this question you can find several options how to handle them if you need to.