Value object or entity

There is two things in my project Advertiser and BonusProgram开发者_运维知识库s.

Business rules are -:

1. Advertisers will select bonus program from list of bonus programs.

2. Only one bonus program can be assigned at a time, previous bonus program will be discarded for that Advertiser.

3. BonusPrograms are not created by Advertiser, only assigned to them.

4. BonusPrograms are not created per advertiser, it is for all advertisers

5. Any new bonusprogram can be introduced at any time in project

My question are -:

1) I have created Bonus program as a separate agg root against Advertiser root aggregate because , advertiser does not create it, it only assigns it. Do I am correct ?

OR

2) Do i make BonusPrograms as valueobject under Advertiser Aggregate because only one bonusprogram is assigned to Advertiser, When a new one is assigned previous one is removed?

I'd go with option 3) which is that BonusProgram is an Entity, but not an Aggregate Root. It's hard to say w/o knowing more of your domain, but from what you described here, Advertising (or Marketing, or soemthing similar) is the aggregate, and Advertisers and Bonus Programs are entities under that aggregate. Not sure what the root would be for the aggregate from what you've said, but it doesn't sound like either Advertisers or Bonus Programs to me.

Firstly, let me say I agree with Paul, and you should accept his answer at some point. I'd have made this a comment but I can express my thoughts this way better.

You are asking two questions here, and there are two common sticky concepts in each.

The first question is the notion of where BonusProgram goes with respect to Advertiser if Advertiser is an aggregate root. This is definitely an interesting question, and depends on your domain and use cases. It doesn't depend on whether BP is a value object or entity though, as aggregates will typically contain both. The point of the aggregate is to simplify object traversal for external (to the aggregate root) objects. You accomplish that by picking one root entity that an external object can have a reference to, and only one (for a use case). This means that a client object can have a reference to Advertiser, but not BonusProgram. Advertiser will hold and necessary references to BP in order to satisfy the clinet object's request.

The second question is whether BP is a value object or an entity. Again the answer depends on your domain. The question to always ask yourself is whether you care about the objects identity or not. If you don't care, it is a value object; if you do care it is an entity. The classic case of a value object is Money - while you certainly care about it, you don't typically care about which dollar is which (one dollar is as wonderful as another)! In this case though, BonusProgram smells more like an entity, and you likely are interesting in knowing things like which BP was in effect last month and what were it's results compared to this month's BP.

Again, Paul is saying the same things and you should accept his answer.

HTH,
Berryl