Spring webflux convert flux to DTO after grouping certain fields into a child DTO object

Question

My rest api is using Spring Webflux. While fetching an Entity from Postgres which is denormalized table, I am getting multiple rows for the same object. While converting to DTO, I need to iterate through multiple User objects for the same user and make a list of all departments he/she is part of and then add the list to the DTO. This way, I can send back a single row per User. How can I achieve this without blocking.

@Entity
public Class User {

    private int id;
    private String name;
    private String department;
    private String departmentArea;

}

public Class UserDTO {
    
    private int id;
    private String name;
    private List<DepartmentDTO> departmentDTO;
    
}

How to I convert the User objects into a UserDTO with a list of DepartmentDTO

Current implementation in service

userRepo.findAllUsersByIds().map(mapper::mapUserDTO));

Answer 1

You can use groupBy operator to group by id and then map every group into a UserDTO

Flux<UserDTO> res = userRepo.findAllUsersByIds()
        .groupBy(user -> user.getId())
        .flatMap(group ->
                group.collectList()
                        .map(users -> {
                            var userDto = new UserDTO();
                            userDto.setId(group.key());
                            userDto.setName(users.get(0).getName());
                            userDto.setDepartmentDTO(
                                    users.stream()
                                            .map(user -> {
                                                var deptDto = new DepartmentDTO();
                                                deptDto.setName(user.getDepartment());
                                                deptDto.setArea(user.getDepartmentArea());
                                                return deptDto;
                                            })
                                            .toList()
                            );
                            return userDto;
                        })
        );