Question

Say we have two aggregate roots in a domain model: Group and User.

Now, Users can be added to or removed from groups. Using the repository pattern, I only modelled the following two interfaces so far:

interface IGroupRepository
{
    Group FindById(int groupId);
}
interface IUserRepository
{
    User FindById(int userId);
    IQueryable<User> GetGroupMembers(int groupId);
    void AddUserToGroup(User user, Group group);
    void RemoveUserFromGroup(User user, Group group);
}

Somehow, the doesn’t feel right. I want to achieve a clean domain model and not end up with a mere data-access layer. What would be a better way to model the above?

EDIT: The root question here seems to be, is it OK with DDD’s guidelines to treat User as a ‘sub-object’ while it at the same time is an aggregate root as well? As I understood DDD, it states that aggregate roots must only be retrieved and stored from one place (the repository) so that’s why I get a bit confused.

Answer 1

If you use a ORM such as NHibernate to it’s full potential you can have methods like this:

class Group
{
    List<User> members;

    void Join(User user)
    {
        members.Add(user);
    }

    void Leave(User user)
    {
        members.Remove(user);
    }
}

A decent ORM will track the changes to the members list and persist those to the database.

This would enable your interfaces to be simplified to:

interface IGroupRepository
{
    Group FindById(int groupId);
}

interface IUserRepository
{
    User FindById(int userId);
}

This should give you some guidance on doing this with NHibernate.