Modeling aggregates

Question

I have the following requirements:

• There are articles created by admin users
• For each articles there are questions related with the article
• The admin can ask between 1 and 3 questions per article
• And each question can have between 2 and 5 answers
• The question text and answer text can be modified in every moment before article be published
• Finally, the admin publish the article and others users can select an answer for each question about the article.

Well, in this situation, you would create an aggregate for article, other for question and other for answer.

If yes, how to keep the invariant that an article have between 1 and 3 questions?

If no, how would you model it?

Thanks a lot!

Answer 1

• Article aggregate includes a set of references to Question (via an aggregate root, e.g. the question ID)
• An attempt to add a reference to a Question fails if there are already 3 references for that Article

Since an aggregate defines a consistency boundary, publishing an article is a saga. Adding a question is likewise going to be a saga if you want the Question to track which Article(s) it's associated with (you didn't directly say that Question maintains an invariant that it's only ever associated with one article).