Simple aggregate root and repository

Question

I\'m one of many trying to understand the concept of aggregate roots, and I think that I\'ve got it! However, when I started modeling this sample project, I quickly ran into

Answer 1

I largely agree with what Sisyphus has said, particularly the bit about not constricting yourself to the 'rules' of DDD that may lead to a pretty illogical solution.

In terms of your problem, I have come across the situation many times, and I would term 'ProcessType' as a lookup. Lookups are objects that 'define', and have no references to other entities; in DDD terminology, they are value objects. Other examples of what I would term a lookup may be a team member's 'RoleType', which could be a tester, developer, project manager for example. Even a person's 'Title' I would define as a lookup - Mr, Miss, Mrs, Dr.

I would model your process aggregate as:

public class Process
{
     public ProcessType { get; }
}

As you say, these type of objects typically need to populate dropdowns in the UI and therefore need their own data access mechanism. However, I have personally NOT created 'repositories' as such for them, but rather a 'LookupService'. This for me retains the elegance of DDD by keeping 'repositories' strictly for aggregate roots.

Here is an example of a command handler on my app server and how I have implemented this:

Team Member Aggregate:

public class TeamMember : Person
{
    public Guid TeamMemberID
    {
        get { return _teamMemberID; }
    }

    public TeamMemberRoleType RoleType
    {
        get { return _roleType; }
    }

    public IEnumerable Availability
    {
        get { return _availability.AsReadOnly(); }
    }
}

Command Handler:

public void CreateTeamMember(CreateTeamMemberCommand command)
{
    TeamMemberRoleType role = _lookupService.GetLookupItem(command.RoleTypeID);

    TeamMember member = TeamMemberFactory.CreateTeamMember(command.TeamMemberID,
                                                           role,
                                                           command.DateOfBirth,
                                                           command.FirstName,
                                                           command.Surname);

    using (IUnitOfWork unitOfWork = UnitOfWorkFactory.CreateUnitOfWork())
        _teamMemberRepository.Save(member);
}

The client can also make use of the LookupService to populate dropdown's etc:

ILookup roles = _lookupService.GetLookup();