Thanks for your post.

For an online vendor’s system, “Address” would be a value object. More than one UserAccount may have an equivalent Address (a husband, wife, and their children might all have separate accounts, but usually share the same address.) In this case, Address has no real conceptual identity. It isn’t important to me as an online vendor to realize that two (or more) Address object instances represent the same physical address. It doesn’t matter to me if a UserAccount provides a new Address I’ve never seen before, or changes their Address, or modifies their street address to change ‘ST’ to ‘STREET’. This is what makes them a value object.

However, if I am writing a system for the electric company, an Address would be an Entity, since it is a unique “endpoint” for which the service can be turned on and off. You can’t have the service on AND off for the same Address. Its identity is important; if you create a new Address, that’s also important.

Anyway, great writeup.

I kind of alluded to that when I mentioned the US Treasury Department. To me I don’t care about the identity of the money I just care about the value it servers me. But in direct contrast and a different context all together the US Treasury care deeply about the identity of the money as it uses the serial number to track against counterfeit bills. So you see the same artifact, in two entirely different scenarios resulting in one context a value object and in the other context a very valuable entity object.

You’ve mentioned to me in the past that an object can be an entity or a value object based on context. What does it look like to transition from an entity to a value object based on context? Or is this even what is meant by that statement?

As far as identity, I think you bring up an excellent point on the characteristics of an entity. Afterall, you wouldn’t want your wife handing you back your son’s wallet, which may contain a lot fewer money value objects. The point is that you are able to properly identify that wallet based on some criteria and have the wallet persisted/retrieved properly by the wife repository based on those characteristics deemed important. In this case, wouldn’t you say that those identifying characteristics are made explicit more so by the repository than the entity itself?

Finally, after playing with ActiveRecord for only a short time, I definitely think it would drive you crazy. AR extends the entities to be persistence aware and although this is great for small applications, it certainly moves them to behave more like business objects.