In what layer in DDD are classes declared as Aggregates roots?

Question

We have a large application that we are starting to develop using DDD. We understand the concept of Aggregates (roots) and bounded contexts and repositories, where repositories accept only aggregate roots to be persisted.

What I am not clear about is this. We have auto generated entity classes for each bounded context in the persistence layer, and have a few domain models sitting in the domain layer. We do this to simplify a very complex database structure (700+ tables), which is why, for instance, we have a Customer entity class and related entities that map to a CustomerModel complex type as a domain model.

The IRepository<IAggregateRoot> has Get, Save, Delete, Update CRUD methods.

Given the example above with Customer and CustomerModel, the dbo.Customer table has about 15 other tables that belong to the same aggreagte. But what classes are actually supposed to implement the IAggreagteRoot?

The aggreagte is a logical grouping, but is it done in the domain:

public class CustomerModel : IAggreagteRoot {}

or is it done in the entity class:

public class Customer : IAggreagteRoot {}

To me it makes more sense to define the domain model classes as aggreagtes, because those are visible to the application(s) "sitting" on top of the domain. The entities are for persistance.

Thank you.

Rafał Łużyński Rafał Łużyński · Accepted Answer · 2015-04-12T07:37:31

I think that you have mixed two concepts here. Everything in domain layer is Your model (domain model): aggregates, entities, value objects etc.

What you named CustomerModel is probably just a DB model (if I understood correctly), and DB models are part of infrastructure level. Domain layer has no knowledge about that.

To answer Your question: Customer should implement AggregateRoot interface.

Example:

infrastructure.ui.show_the_oldest_user:

oldest_user_dto = user_service.get_the_oldest_user()

oldest_user_dto is just a raw data

application.service.get_the_oldest_user:

user = user_repository.get_oldest()
return user

user is an aggregate root

infrastructure.persistence.repository.user.get_oldest:

orm_user = user_orm_model.order('age').desc().one()
user = this.reconstitute(orm_user)
return user

orm_user is an instance of user model from DB, which we have to map to our aggregate root (reconstitute).

This way we have no orm instance flying across all layers which is a really bad coupling. I also map aggregate to dto when I return it from application service, so UI is not coupled to domain model, but it's not a must.

In what layer in DDD are classes declared as Aggregates roots?

2 Answers