Domain-Driven Design in PHP

Reference Other Entities by Identity

When two Entities don't form an Aggregate but are related, the best option to have Entities reference one another is by using Identities. Identities were already explained in the Chapter 4, Entities.

Consider a User and their Orders, and assume we haven't found any true invariant. User and Order wouldn't be part of the same Aggregate. If you need to know which User owns a specific Order, you can probably ask the Order what its UserId is. UserId is a Value Object that holds the User Identity. We get the whole User by using its Repository, the UserRepository. This code generally lives in the Application Service.

As a general explanation, each Aggregate has its own Repository. If you've fetched a specific ...