How to model aggregates with one to zero-or-many relationship

Question

I'm reading Implementing Domain Driven Design book of Vaughn Vernon. But I soon get into trouble for modeling aggregates in my domain - football matches.

My case is list all matches based on some conditions (filters), for ex:

• list all matches of a league that will take place
• list all matches of a league in a group
• list all matches of a league in knockout stage

As I see a league (ex. World Cup) will have many stages such as group stage or knockout stage.

A stage will have many rounds, ex. Round of 16, Quarter-finals, etc.

A round will have many matches.

However, some leagues do not have stage, ex. Premier League. Even Friendly International leagues do not have both stage and round.

First solution:

We will have 4 aggregates:

• League will contain Stages
• Stage will contain Rounds
• Round will contain Matches

For leagues do not have Stage or Round, a special identifier will be created for Stage and Round, ex. NO_STAGE or NO_ROUND

Second solution:

League will contain Matches. Stage or Round are just attributes (Value Objects) of Match

Third solution:

Create all types of League:

• League have stages, rounds and matches
• League have rounds and matches
• League have only matches

Answer 1

The underlying problem is, that you are trying to model data instead of behavior. You are creating a relational model (what contains what, one-to-one, one-to-many, etc.), not an object model. Modeling data is meaningless without a context of what functionality it must support.

If you do only want a data model, then DDD is irrelevant since that is not a data modelling technique.

I assume you want to build object-oriented software though. In this case, approach the problem by identifying what functionality you want from your software, and model that.

For example: schedule matches, prepare/display a calendar of events, simulate progression, etc.