Convert and collect Flux as database entities and save to database, after return in controller

Question

I am migrating to the reactive stack and I am having difficulties working with the reactive approach.

The task is to get data from a remote service in the Flux<GroupDTO> format, convert each GroupDTO and collect them into a List<Community>, then create a Communities object based on a previously collected list and save this data to a Postgresql reactive database. I ask for help.

GroupController:

@GetMapping("/groups")
public Flux<ResponseEntity<GroupDTO>> getGroups(@RequestParam("q") @Max(value = 128) String q) {

    Flux<GroupDTO> flux = groupService.getGroups(q); // getting data from remote service

    // how to convert and collect all groups to reactive database entity and save to reactive database(postgresql) here?

    return flux.map(ResponseEntity::ok)
            .defaultIfEmpty(ResponseEntity.notFound().build());;

}

GroupDTO:

@Jacksonized
@Builder
@Value
@JsonIgnoreProperties(ignoreUnknown = true)
public class GroupDTO {

    int id;

    @JsonProperty("is_member")
    boolean member;

}

Class Communities with all converted and collected from Flux<GroupDTO>:

@Getter
@NoArgsConstructor(access = AccessLevel.PROTECTED, force = true)
public class Communities {

    @Id
    private Long id;

    private List<Community> elements;

    private String q;

    //@CreationTimestamp
    @JsonSerialize(using = CustomTimestampSerializer.class)
    private Timestamp timestamp;

    public Communities(String q, List<Community> elements) {
        this.q = q;
        this.elements = elements;
    }

}

Each GroupDTO may convert to Community:

@Getter
@NoArgsConstructor
public class Community {

    @Id
    private Long id;

    private Long groupId;

    public Community(Long groupId) {
        this.groupId = groupId;
    }

}

Reactive repositories:

@Repository
public interface CommunitiesRepository extends ReactiveSortingRepository<Communities, Long> {}

@Repository
public interface CommunityRepository extends ReactiveSortingRepository<Community, Long> {}

The names of the Community and Communities database models were chosen as a result of the fact that earlier when working with JPA Hibernate (Postgresql) errors occurred due to the keywords "Group" / "Groups". If there is a way, then I would be happy to fix it in the current migration.

If there are errors in the design, I will be glad to hear tips on how to fix them.