@Entity()
export class Candidate {
    @PrimaryGeneratedColumn()
    id: number;
    
    ...
    @ManyToMany(() => Language, { nullable: true })
    @JoinTable()
    languages: Language[];
    @ManyToMany(() => Tool, { nullable: true })
    @JoinTable()
    tools: Tool[];
    @ManyToMany(() => ActivityArea, { nullable: true })
    @JoinTable()
    activity_areas: ActivityArea[];
    ...
    @Column({ nullable: true })
    desired_starting_date: string;
    ...
}

@Entity()
export class Language {
    @PrimaryGeneratedColumn()
    id: number;
    @Column({unique: true})
    name: string;
}

async update(id: number, updateCandidateDto: UpdateCandidateDto) {
    try {
        const candidate = this.generateaCandidate(updateCandidateDto); // this is the new value of candidate
        return await this.candidateRepository.update({
            id
        }, {
            languages: candidate.languages,
            tools: candidate.tools,
            activity_areas: candidate.activity_areas,
            desired_starting_date: candidate.desired_starting_date
        });
    } catch (error) {
        throw new HttpException(error.detail, HttpStatus.INTERNAL_SERVER_ERROR);
    }
}

async update(id: number, updateCandidateDto: UpdateCandidateDto) {
    try {
        const new_candidate = await this.generateCandidate(updateCandidateDto);
        for (const prop in new_candidate) {
            if (typeof new_candidate[prop] == 'object') {
                const actualRelationships = await this.candidateRepository
                    .createQueryBuilder()
                    .relation(Candidate, prop)
                    .of(id)
                    .loadMany();
                await this.candidateRepository
                    .createQueryBuilder()
                    .relation(Candidate, prop)
                    .of(id)
                    .addAndRemove(new_candidate[prop], actualRelationships);
            } else {
                const json = JSON.parse(`{ "${prop}" : "${new_candidate[prop]}" }`);
                this.candidateRepository
                    .createQueryBuilder()
                    .update(Candidate)
                    .set(json)
                    .where('id = :id', {id: id})
                    .execute();
            }
        }
        return await this.findOne(id);
    } catch (error) {
        console.error(error);
        throw new HttpException('Update failed', HttpStatus.INTERNAL_SERVER_ERROR);
    }
}