如何定义聚合的一对多关系

Question

I have two aggregates Blog and Post with a one-to-many relationship ( Blog is related to several Post ). According to domain driven design best practices I am unsure if Blog should hold a collection of Post references:

public class Blog : Entity<BlogId>, IAggregateRoot
{
    IEnumerable<Post> posts;
}

when Post also has an indirect id reference through BlogId :

public class Post : Entity<PostId>, IAggregateRoot
{
    BlogId BlogId;
}

If more context is required in order to answer the question please let me know.

Answer 1

In the book by Vaughn Vernon, Implementing Domain-Driven Design, you can find a entire chapter on how to define your aggregates and some rules of thumb you can apply.

One of them is to try to keep your aggregates as small as possible. In your case, I would opt to avoid having a list of Post objects inside your Blog object. Imagine that your Blog object has an attribute name and you just want to change it: when you will retrieve that object from the database, if it contains all of the Posts, you will retrieve them too, making an unnecessary join for the use case you are working on, and that will decrease the performance.

From a design point of view, probably that Blog is not defined by its Post, so actually you don't need them inside it. An aggregate should be responsible of all of its invariant rules (business logic rules) and no more.

My advice is to just keep it as simple as possible and always focus on solid principles.

Good luck!

Answer 2

DDD does not tell you how to implement your application, but gives you just guidelines. Holding a list of "posts" into a "blog" class and a logic reverse refernce is definetly ok if you cannot do something else, but the most important thing to understand is how a "blog user" will be able to act upon posts.

Thus, feel free to implment the domain model in your most preferred way, but the only way to add posts, delete post, etc will be through commands sent to the "blog" entity, so that will be the blog itself to grant you that the posts are wired correctly according with your implementation

Answer 3

A rule for aggregates is that you should reference an aggregate by its aggregate root id, so in Blog aggregate you should keep a list of Post ids.