Basic Aggregate Question

Question

Is client code allowed to reference entities within an aggregate that is not the root? I have a Story (Root), Team (Entity) and TeamMember

Answer 1

My opinion - it should not. Having a reference to an entity that belongs to certain aggregate means that you are able to invoke a method on that entity without full aggregate context and if you allow that, you can never be sure if your entire aggregate is valid and consistent.

Simple example:

    public class MyAggregateRoot
    {
        protected MyEntity entity;

        public void BuildUpAggregate()
        {
            ValidateSomeRule();
            LoadEntityFromDatabase();
        }

        public MyEntity MyEntity
        {
            get 
            {
                VerifySomeOtherRule();
                return entity; 
            }
        }
    }

As you can see, while building and retrieving MyEntity via aggregate root we have two validation rules checked - if you would allow the client to reference MyEntity directly, the aggregate might change in time between client retrieved the entity and performed an operation on it, so the checks would no longer be valid, but you wouldn't event be aware of this fact. In other words, your aggregate would be inconsistent and potentially invalid.

In general, in DDD there's a strong rule that states that all access to entities in aggregate should be performed by traversing from the aggregate root - this rule is exactly for the sake of consistency of aggregates.

That said, what your client can reference is a projection of an entity - a sort of read-only copy that contains only data required for displaying and deciding whether a certain action is available in current context. You can go even further and aggregate the data from a set of entities to a single projection, tune it up a little, so it would be tailored to the requirements of UI. I found this technique quite useful for not allowing my UI to decide how the business domain should be modeled.