Entity contain another entity

[yookky]

Hi!
I read in the description in the section about entity:
Can contain other objects, such as other entities or value objects.

Can u explain how to implement this?
Or is this decision correct?
example:

export class UserEntity extends AggregateRoot < UserProps > {
    protected readonly _id: AggregateID;
    protected _userBook: UserBookEntity;
    get(): UserBookEntity {
        return this._userBook
    }
    set(userBook: UserBookEntity) {
        this._userBook = userBook;
    }
    static create(create: CreateUserProps): UserEntity {
        ....
    }
}

And UserBookEntity the same entity like UserEntity.
Thank you!

[Sairyss]

Your example is partially correct.

1. If your entity contains other entities, it is called an Aggregate Root, and all manipulations with child entities must be performed through the aggregate root. In your example you have a get() method that returns a child entity, but it makes it possible to violate the rule like this:

const userBook = userEntity.get(); // <-- returns UserBookEntity
userBook.doSomething() // <-- aggregate integrity violated

You should not return child entities from an Aggregate Root (you can return it's data though if you need to, but it must be immutable).

If you need to perform manipulations to child entities, do this instead:

userEntity.doSomethingToBook(); // <-- method is called on UserEntity, not the it's child

This way you don't cross any boundaries, no risk to execute something in a child entity that depends on a parent.

2. All child entities must be closely related (same bounded context) to your parent entity. In your case, book is not the same bounded context as a user, so I would not put it inside a user. But the principle is correct, you can have a child entity as a property inside some other entity (same as value objects).

Some more rules of Aggregate Roots here

[yookky]

Thanks a lot!