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I am working on an application where users can follow each other, in a similar fashion to Twitter.
After reading up on DDD, I understand that my users are Entity Objects - I refer to them using their unique ID.
When one user 'follows' another (i.e. forms a Connection), the relationship is stored in a many-to-many table. Its fields include FollowerID, TargetID, and Status. There can be only two records for each Follower/Target combination (one Active, the other Inactive), so I can safely identify objects based on their attributes.
So, I think my Connection objects are Value Objects, not Entity Objects, but I'm not sure. Can you help me with this decision?
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The main difference between entities and value objects lies in the way we compare their instances to each other. The concept of identifier equality refers to entities, whereas the concept of structural equality - to value objects. In other words, entities possess inherent identity while value objects don't.
Value Object is an object that represents a concept from your problem Domain. It is important in DDD that Value Objects support and enrich Ubiquitous Language of your Domain. They are not just primitives that represent some values - they are domain citizens that model behaviour of your application.
DTO is a class representing some data with no logic in it. On the other hand, Value Object is a full member of your domain model. It conforms to the same rules as Entity. The only difference between Value Object and Entity is that Value Object doesn't have its own identity.
A domain entity in DDD must implement the domain logic or behavior related to the entity data (the object accessed in memory). For example, as part of an order entity class you must have business logic and operations implemented as methods for tasks such as adding an order item, data validation, and total calculation.
You are correct that entities are unique and carry the notion of having an identity (i.e. only one unique user can exist). A Connection is dependent on other User entities. It represents some aspect between two users. That aspect is whether there is an active or inactive connection. Without containing the data of which users are connecting, a connection has no identity. It may even have it's own primary key in the database, but from a domain perspective, it has no identity of it's own.
Therefore, I would say that Connection is a value object.
To support my conclusion, Microsoft.Net Architecting Applications for the Enterprise, page 187, says:
A value object class represents an entity in the domain that mostly contains data and lives for the data it contains. A value object is fully identified by a combination of values it contains. An entity object, on the other hand, has its own life and rich behavior regardless of the data it contains. Entity objects are usually objects with a longer lifetime. A value object represents an aspect of an entity and can live only in relation to an entity.
And also on page 189:
One further comment is needed to explain the difference between entities and value objects. You don’t need a repository or a data mapper for a value object. You need a repository only for an entity. The repository (or the mapper) for a given entity will certainly take care of all value objects that depend on a given entity.
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