DDD: Connection objects are Entity Object or Value

问题:

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?

回答1:

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.

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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.

回答2:

Some time ago, I saw a cartoon about scientist that had invented cloning. Every time he cloned himself, he destroyed previous version. Then person that was watching demonstration decided to interrupt and sabotaged destruction part so there were two scientists. Cartoon ended with some interesting existential questioning.

Values vs entities is not about having or not having id fields in one or another form. Point is - how we are looking at those objects through our domain perspective. If they are value objects, then only their value matters - 1st, 3rd and 53rd scientist are the same. If we care about identity, if we think that cloning 3rd scientist will never be like 1st one, then our object is an entity.