Aggregates that require sharing of an entity

Question

Consider my scenario of a model consisting of two aggregate roots, Customer and Order as well as a "shared" entity Address.

Also note that Address is abstract has the following subclasses: PhysicalAddress, PostOfficeBoxAddress and PrivateBagAddress.

A Customer can have many addresses organized into some sort of address book. Upon making an order a customer would select an Address from their address book to be used as a delivery address.

My initial thoughts were to share an address between the two entities, but I have since opted out as it will cause trouble with managing the respective invariants.

Another option I could go for is to create two hierarchies of Address, each for their purposes as a customer address or delivery address. This again doesn't seem right as there is a lot of repeated code.

How would I model this situation properly?

Answer 1

An entity is something that should be able to exist by ifself such as a customer or order. However an Address is not an entity, an Address is a value type and cannot exist on its own hence:

1. An Address can only exist as a value type that is aggregated within an entity
2. You can define your address hierarchy as a value type in your domain but many entities may use this.

We find that we come across these types of entities all the time such as Address, MoneyType etc.

Solution would be to create 1 Address hierarchy value type in your domain. Then any entity can have an Address as a property where applicable