I’m encountering an error while saving a Department object with Spring Data R2DBC. My Department model has an employees property that can hold a list of Employee objects. However, when I try to create a new department with an empty employee list, I get the following error:

java.lang.IllegalArgumentException: Unsupported array type: io.spring.graphql.employeedemo2.model.Employee

Code Snippet:

Department.class

@Data
@NoArgsConstructor
@Table(name = "department_tbl")
public class Department {

    @Id
    @Column("department_name")
    private int id;

    @Column("department_name")
    private String name;

    private List<Employee> employees;

    public Department(int id, String name, List<Employee> employees){
        this.id = id;
        this.name = name;
        this.employees = new ArrayList<>();
    }

AddDepartmentInput.class

@Data
@NoArgsConstructor
@AllArgsConstructor
public class AddDepartmentInput {

    private int id;
    private String name;

}

@MutationMapping
public Mono<Department> createDepartment(@Argument AddDepartmentInput departmentInput) {
    return Mono.just(departmentInput)
        .map(dptInput -> {
            Department department = new Department();
            department.setId(dptInput.getId());
            department.setName(dptInput.getName());
            department.setEmployees(new ArrayList<>()); // Empty list initialization
            return department;
        })
        .flatMap(department -> departmentRepository.save(department));
}

and this is my schema.graphqls

type Employee {
    id: ID!
    name: String
    salary: Float
    departmentId: ID!
}

type Department {
    id: ID!
    name: String
    employees: [Employee]
}

type Mutation {
    createDepartment(departmentInput: AddDepartmentInput ):Department
}

input AddDepartmentInput {
    id:ID!
    name:String
}

Context:

• I’m using Spring Data R2DBC for data persistence.
• I want to allow creating departments initially without any associated employees.

Question:

How can I effectively save a Department object with an empty employees list using Spring Data R2DBC? Are there recommended approaches for handling empty lists within models?