Map entity relationships

[betelgense]

If I have two aggregate root "article" & "tag" and I need to have a relationship by composition having the direct reference how it should be modeled

class Article {
// ...
tag: Tag; // Direct reference to a Tag entity
}

Can anyone provide a minimal example if I were to change the business rules to that a user can have 1...n wallets and a wallet can have 1...n users

Thank you!

[Sairyss]

If I have two aggregate root "article" & "tag"

In case of Article and Tag, only Article is an aggregate root, tag is just an Entity.

Can anyone provide a minimal example if I were to change the business rules to that a user can have 1...n wallets and a wallet can have 1...n users

You are thinking of domain entities as database tables. You shouldn't. Your domain entities are not tables and should be designed differently.

Domain model could look like this:

// user domain entity/aggregate
class User {
  id: string;
  // ... rest
}

// wallet domain entity/aggregate
class Wallet {
  id: string;
  userId: string; // you most likely don't need an actual relation here, just a user id
  // ... rest
}

Database model:

CREATE TABLE users (
   id  CHARACTER VARYING PRIMARY KEY NOT NULL,
   -- rest
);

CREATE TABLE wallets (
   id CHARACTER VARYING PRIMARY KEY NOT NULL,
   user_id CHARACTER VARYING 
   -- rest
);
-- add relation on the database level
alter table "wallets" add constraint "wallet_user_id_fk" foreign key ("user_id") references "users" ("id")

[betelgense]

I had modeled "tag" as an aggregate since it seems to have an important relationship with users and affects experience recommendations. It also has other related entities like segments, groups, etc.

I saw that it doesn't express me very well but the question was how to persist (with Slonik) following the good practices that are recommended here. For a practical example "tag" will be an entity part of the aggregate of article:

article.entity.ts

export interface ArticleProps {
  title: string;
  tags?: Tags[];
  // ...more props
}

export class ArticleEntity extends AggregateRoot<ArticleProps> {
  protected readonly _id: AggregateID;

  addTag(tag: Tag): void {
   // this.addEvent(...
  }

   /* ...more code */
}

article.repository.ts

export const articleSchema = z.object({
  id: z.string().uuid(),
  createdAt: z.preprocess((val: any) => new Date(val), z.date()),
  updatedAt: z.preprocess((val: any) => new Date(val), z.date()),
  title: z.string().min(1).max(255),
});

export type ArticleModel = z.TypeOf<typeof articleSchema>;

@Injectable()
export class ArticleRepository
  extends SqlRepositoryBase<ArticleEntity, ArticleModel>
  implements ArticleRepositoryPort
{
  protected tableName = 'articles';

  protected schema = articleSchema;

  constructor(
    @InjectPool()
    pool: DatabasePool,
    mapper: ArticleMapper,
    eventEmitter: EventEmitter2,
  ) {
    super(pool, mapper, eventEmitter, new Logger(ArticleRepository.name));
  }

 /* ...more code */
}

tag.entity.ts

export interface TagProps {
  name: string;
   // ...more props
}

export class TagEntity extends Entity<TagProps> {
  protected readonly _id: AggregateID;
  
  /* ...more code */
}

tag.repository.ts

export const tagSchema = z.object({
  id: z.string().uuid(),
  createdAt: z.preprocess((val: any) => new Date(val), z.date()),
  updatedAt: z.preprocess((val: any) => new Date(val), z.date()),
  name: z.string().min(1).max(255),
});

export type TagModel = z.TypeOf<typeof tagSchema>;

@Injectable()
// export class TagRepository extends SqlRepositoryBase<TagEntity, TagModel> 
// SqlRepositoryBase expects an aggregate root 
export class TagRepository {
...
}

So my solution was to create a repo for my ternary table where the "article_tag" relationships are located. Once a new tag is added it would look something like this:

add-tag-to-post.service.ts

@CommandHandler(AddTagToPostCommand)
 ...more code

article.addTag(tag);
await this.articleRepo.save(article);

 ...more code

Something like this would look like the updated article repo
article.repository.ts

private saveTags (tags: Tags) {
    return this.tagRepo.saveBulk(tags);
}

async save(article: Article): Promise<void>  {
 ...validations

 await this.saveTags(article.tags);

  ...more code
}

Then it only remains to persist the relationship in the ternary table in a similar way to saveTags. Would this be correct to put it that way?

Thanks in advance. Maybe there are errors in the code because I try to simplify it as much as possible

[salsaly4]

IMHO, Tag here seems to be more of a Value Object, not entity.

It's described here - https://github.com/Sairyss/domain-driven-hexagon#value-objects

[betelgense]

Not really, tags can be modified and need to be referenced in all post and other places, like sub-tags, tag-votes, etc

[Sairyss]

@betelgense your code seems fine. All manipulations to the tags should go through an aggregate root (article), and saving of an entire aggregate should be done by executing a transaction in aggregate root repository (article repository), which you did.

Though in the original question you asked about "User" and "Wallet", which is a totally different use case than "Article" and "Tag", and I personally would not put "Wallet" inside a "User". "Tag" may be fine because it's relatively small and seems to fit well as part of an "Article", but "Wallet" seems like a totally separate domain from a "User".