How to deal with child entities

[Anxium]

Hi everyone,

Firstly, thanks Sairyss for this amazing repository, it's very helpful as I'm new to the DDD concepts.

I'm currently trying to build an application with "Member" as an aggregate root and "MemberEmail" as a child entity.

From what I understand, the "MemberEmail" should extends the Entity class and then, in the create-service of the "Member" entity I have to create the "MemberEmail" entities as well.

Here is the create-member.service.ts:

@CommandHandler(CreateMemberCommand)
export class CreateMemberService implements ICommandHandler {
	constructor(
		@Inject(MEMBER_REPOSITORY)
		protected readonly memberRepo: MemberRepositoryPort,
	) {}

	async execute(
		command: CreateMemberCommand,
	): Promise<Result<AggregateID, MemberAlreadyExistsError>> {
		const member = MemberEntity.create({
			firstName: command.firstName,
			lastName: command.lastName,
			gender: command.gender,
			address: new Address({
				country: command.country,
				city: command.city,
				postalCode: command.postalCode,
				street: command.street,
				number: command.number,
				box: command.box,
			}),
			emails: command.emails.map((e) => {
				return MemberEmailEntity.create({
					email: e.email,
					isDefault: e.isDefault,
				});
			}),
		});

		try {
			await this.memberRepo.transaction(async () =>
				this.memberRepo.insert(member),
			);
			return Ok(member.id);
		} catch (error: unknown) {
			if (error instanceof ConflictException) {
				return Err(new MemberAlreadyExistsError(error));
			}
			throw error;
		}
	}
}

My issue is that I can't figure out how to save it into my database, as I want to save the member emails in an other SQL table (meaning I also need the memberId)
Should I use events on the member creation or create an other repository (But if I'm doing this it means that I have to extends the AggregateRoot class from the "MemberEmail" entity instead of the Entity class ? How would you do it ?

Thanks a lot for your time.

[Sairyss]

Email seems like a Value Object to me, it's a property of a Member, not an entity with a separate identity. So I'd say extend ValueObject.
For saving it to the database, your memberRepo can save the email in a separate table, with the same ID as a member ID (many to one relation). When you query you just join 2 tables.

One of the most important things to understand about DDD is that your domain model is not the same as database model. You shouldn't create an entity for something just because it will be in a separate table. You can just use it as a regular value, or as a value object, and let repository decide how it wants to save it.

[Anxium]

Thanks for your quick answer, really appreciate it. :)

So from what you said, this is what I understand:

• I should convert the "MemberEmail" from an entity to a value object
• I can create the relations from the member repository

And does that mean that if I want to retrieve the ID of each email, I can add it in the response dto/queries? Because (for example) if I need to update an email, I'll need the ID or memberId & email from your perspective? Or maybe I don't need an auto-generated ID on my member_email table?

Again, thanks for your time.
I'm trying to understand as much as I can 😄 I think everything will be clearer once I understand that

[Sairyss]

Email does not need its own id, you have a memberID + email to update it UPDATE emails SET email = '[email protected]' WHERE email = '[email protected]' AND memberId = '123';

[Anxium]

Got it, thanks a lot !

[tin-pham]

When I have child entity, i consider:

• if that entity can live along without the aggregate root, if it can, it should have it own aggregate
• if that entity value are 2 difference thing even it look the same, for example 2 car look the same, but the number plate is difference, then it difference. In your case if 2 email have the same value, they are not difference than another => Value Object

[Anxium]

Thanks 👍

[ElliottCuvelier]

@Sairyss First of all thank you for this awesome repo 🙏

How should the mapping be handled in this use case if you want to create a MemberEmail for example? Once it's added to the Aggregate Root, how should it be handled by the repository?

• Should it call createMemberEmail on the repository and pass the MemberEntity so that it can be mapped toPersistence and then filter out the new one so that it can be saved in DB?
• Should it call createMemberEmail on the repository and pass the new MemberEmail and map it in the repository to the MemberEmail DbModel and saved in DB?
• Should it call createMemberEmail on the repository and pass the new MemberEmail and map with a specific toPersistence mapper for MemberEmail?

[Sairyss]

You can map it in a mapper or right in the repository, depending on your preference. For example:

class MemberMapper {
  toPersistence(entity: MemberEntity): MemberModel {
    const props = entity.getPropsCopy();
    const record: MemberModel = {
      id: props.id,
      createdAt: props.createdAt,
      updatedAt: props.updatedAt,
      email: props.email.value, // <- extract email value from the value object
    };
    return record;
  }
}

Then you save it like this: memberRepository.insert(member);
You don't need a separate method for saving email, it is a part of a member and is saved together with it.

[Kainar01]

@Sairyss Thanks for this fantastic repo!

I have 2 questions on persisting entities.

1. In the example above, would I create memberRepo.create method where I insert Member using memberRepo.insert method and insert MemberMails using memberRepo.writeQuery ?
2. What if I want to bulk update/insert entities. Do I always have to use create/update method of entity, which would generate multiple SQL queries? Or can I use SQL bulk insert queries to drastically improve performance? ( I have read that

it's acceptable to prioritize performance over following design patterns strictly

in Patterns of Enterprise Application Architecture by Martin Fowler.

[Sairyss]

@Kainar01

1. yes, something like that, if that makes sense in your case. Insert it as a part of a transaction
2. it's more efficient to use bulk inserts, so yes you can create a bulk insert method