DDD shared entity between two aggregate roots

Question

I'm working with two different aggregate roots: Post and Question. Both of them have a Category.

So far I have implemented it as a shared entity (which I'm not sure if is a correct design in DDD).

public class Post
{
    public Guid Id { get; private set; }

    public Category Category { get; private set; }

    public string Title { get; private set; }

    public string Body { get; private set; }
}

public class Question
{
    public Guid Id { get; private set; } 

    public Category Category { get; private set; }

    public string Title { get; private set; }

    public string Body { get; private set; }
}

public class Category
{
    public int Id { get; private set; }

    public string Name { get; private set; }

    public string Key { get; private set; }
}

Note: I'm aware I'm falling into primitive obsession anti-pattern, and I have plans on refactor the primitives into ValueObjects.

After read this post DDD: Share entity with multiple aggregate roots I'm thinking that maybe I should convert the Category in a ValueObject (with multiple fields).

In theory Category could be an Entity with its own lifecycle, but reality is that I don't really add/remove/update categories.

Is it possible to use a shared Entity on DDD? Or I better rather use a ValueObject?

anick chowdhury anick chowdhury · Accepted Answer · 2019-09-30T20:56:28

Lets deal with one aggregate first: Post

Now to answer your question:

Is it possible to use a shared Entity on DDD? Or I better rather use a ValueObject?

It depends on what you will do with Category.

Scenario 1:

You have a feature(or page) in your application to show all posts of a category. I would go with the following design:

public class Category
{
    public int Id { get; set; }
    //this is my in-memory database. Use repository and service to adjust yours
    public static List<Post> Posts;

    public Category()
    {
        Posts = new List<Post>();
    }

    public void AddPost(Guid id, string title, string body)
    {
        var post = new Post(id, title, body, this.Id);
        //saving the post into in-memory. Perhaps you can check some business logic inside Post entity
        Posts.Add(post);
    }

    // You can retrieve all posts of a single category
    public IEnumerable<Post> GetAllPosts()
    {
        return Posts.Where(x => x.CategoryId == this.Id);
    }
}

public class Post
{
    public Guid Id { get; private set; }
    public string Title { get; private set; }
    public string Body { get; private set; }
    public int CategoryId { get; private set; }

    public Post(Guid id)
    {
        Id = id;
    }

    public Post(Guid id, string title, string body, int categoryId)
    {
        //I prefer to pass guid into domain from external services.
        //Using this way, your service will have the id to return to upper layers.
        //Alternatively you can create new guid here on your own
        Id = id;
        Title = title;
        Body = body;
        CategoryId = categoryId;
    }

    // you can retrieve a post detail
    public Post GetPost()
    {
        return Category.Posts.FirstOrDefault(x => x.Id == this.Id);
    }
}

I can see only one aggregate root in this scenario: Category.

Scenario 2:

You have posts page, from there users can view detail post. Additionally, every post has a category which will be shown somewhere on that detailed page. You can have following simple design:

public class Post
{
    public Guid Id { get; private set; }
    public string Title { get; private set; }
    public string Body { get; private set; }
    public string CatKey { get; private set; }

    public Post(Guid id)
    {
        Id = id;
    }

    public Post(Guid id, string title, string body, string catKey)
    {
        //I prefer to pass guid into domain from external services.
        //Using this way, your service will have the id to return to upper layers.
        //Alternatively you can create new guid here on your own
        Id = id;
        Title = title;
        Body = body;
        //I don't even bother with category id. This is a simple value object, you can store all of your categories
        //into a hashtable of key-value
        CatKey = catKey;
    }

    // you can retrieve a post detail
    public Post GetPost()
    {
        //get your post detail from repo
    }
}

Hope you can make your decision now.

DDD shared entity between two aggregate roots

2 Answers