why some entities in equals(..) use many properties and not only their ID?

[gt4dev]

Hi

Vaughn, could you explain it? I'd appreciate ;] For example Product: uses ProductId and TentatId.

public class Product extends Entity {

    private Set<ProductBacklogItem> backlogItems;
    private String description;
    private ProductDiscussion discussion;
    private String discussionInitiationId;
    private String name;
    private ProductId productId;
    private ProductOwnerId productOwnerId;
    private TenantId tenantId;

    // ..

    @Override
    public boolean equals(Object anObject) {
        boolean equalObjects = false;

        if (anObject != null && this.getClass() == anObject.getClass()) {
            Product typedObject = (Product) anObject;
            equalObjects =
                this.tenantId().equals(typedObject.tenantId()) &&
                this.productId().equals(typedObject.productId());
        }

        return equalObjects;
    }
}

Entity is identified by its ID (ProductId) thus:

• shouldn't ProductId be the only property used in equals(..)?
• isn't using more properties in equals(..) breaking rule that entities are identified (compared, matched) only by ID?

Thank you GT

BTW. Great book. I'm reading it 2nd time ;]

[gt4dev]

I found answer in IDDD book on page 185.

It allows to identify entities with strategy of 'late identity assignment', before main identity is assigned. It's VO-like equals. Especially useful in Sets.

[VaughnVernon]

@gt4dev No. It's because the tenantId and productId are a composite id, but not held as one. In a SaaS with multiple tenants all entities must be "striped" with the tenantId. They are separated so the tenant is clearly identified and can be queried on its own.

[gt4dev]

I know, it's just a sample but as identity is top important in entities let me ask sth.

Generalizing IDDD sample code: When I 1st time saw Thing with ThingId then I thought that ThingId is just id of thing [common sense] But it's not (as ID needs TenantId)

From my PoV clearer would be [in saasovation context]

1. either create VO ThingId - which hides all inner elements: id of particular thing + all striping id-s [tenant, product ...] etc
2. or rename ThingId to sth like ThingRawId or ThingUnstripedId - to explicitly express in name that it's not real id

What do think you about it, especially point 1?

IMO with "1"

• model [and code] is simpler
• still you can do all searches for "things" by tenant etc. as is done in current IDDD sample It's even more cleared when ID is more complex [like in eg BacklogItem where ID is build from 3 IDs].

BTW It's late answer, but in the meantime I've been sharpening my DDD skills ;] Doing study notes https://github.com/gt4dev/study-notes

Your and Evans books are big big changers. Thanks!