How do you model roles / relationships with Domain Driven Design in mind?

Question

If I have three entities, Project, ProjectRole and Person, where a Person can be a member of different Projects and be in different Project Roles (such as "Project Lead", or "Project Member") - how would you model such a relationship?

In the database, I currently have the following tablers: Project, Person, ProjectRole Project_Person with PersonId & ProjectId as PK and a ProjectRoleId as a FK Relationship.

I'm really at a loss here since all domain models I come up with seem to break some "DDD" rule. Are there any 'standards' for this problem?

I had a look at a Streamlined Object Modeling and there is an example what a Project and ProjectMember would look like, but AddProjectMember() in Project would call ProjectMember.AddProject(). So Project has a List of ProjectMembers, and each ProjectMember in return has a reference to the Project. Looks a bit convoluted to me.

update

After reading more about this subject, I will try the following: There are distinct roles, or better, model relationships, that are of a certain role type within my domain. For instance, ProjectMember is a distinct role that tells us something about the relationship a Person plays within a Project. It contains a ProjectMembershipType that tells us more about the Role it will play. I do know for certain that persons will have to play roles inside a project, so I will model that relationship.

ProjectMembershipTypes can be created and modified. These can be "Project Leader", "Developer", "External Adviser", or something different.

A person can have many roles inside a project, and these roles can start and end at a certain date. Such relationships are modeled by the class ProjectMember.

public class ProjectMember : IRole
{
    public virtual int ProjectMemberId { get; set; }
    public virtual ProjectMembershipType ProjectMembershipType { get; set; }

    public virtual Person Person { get; set; }
    public virtual Project Project { get; set; }
    public virtual DateTime From { get; set; }
    public virtual DateTime Thru { get; set; }
    // etc...
}

ProjectMembershipType: ie. "Project Manager", "Developer", "Adviser"

public class ProjectMembershipType : IRoleType
{
    public virtual int ProjectMembershipTypeId { get; set; }
    public virtual string Name { get; set; }
    public virtual string Description { get; set; }

    // etc...
}

Answer 1

Aren't you confusing the "Description" of a role with the role a person has in a project? Adding the "RoleDescription" concept (a 'role-class' so to speak), and "RoleInstance" objects referring to actual persons in projects may help.

Answer 2

What you have is a many-to-many relationship with additional data, the role. We have a similar structure except in our case a person may have multiple roles on a project, so I struggled with the same questions. One solution is to create a ProjectPerson class that extends Person and adds the role property:

public class ProjectPerson : Person
{
    public string Role { get; set; }
}

Your Project class now has a collection of ProjectPerson but the Person class has a collection of Project because it doesn't make sense to extend the Project class to add role. You'll have to do some additional work (look up the Person in the ProjectPerson collection) to find the role on a Project from the Person's perspective.

A second solution is the standard way to handle many-to-many relationships with additional data. Create a ProjectRole class and model it as the many side of two one-to-many relationships from Project and Person. That is, both Project and Person each have a collection of ProjectRole.

It's important to consider how well your data access strategy will support the model in choosing a solution. You want to avoid scenarios where loading the collection requires one or more trips to the database for each object in the collection.

Answer 3

Here's how I would handle it:

class Person
{
  string Name { get; set; }
  IList<Role> Roles { get; private set; }
}

class Role
{
  string Name { get; set; }
  string Description { get; set; }
  IList<Person> Members { get; private set; }
}

class Project
{
  string Name { get; set; }
  string Description { get; set; }
  IList<ProjectMember> Members { get; private set; }
}

class ProjectMember
{
  Project Project { get; private set; }
  Person Person { get; set; }
  Role Role { get; set; }
}

The ProjectMember class brings them all together. This model gives you the flexibility to assign the same Person to different Projects with different Roles (e.g. he might be a Developer on ProjectA, and a Tester on ProjectB).

Please don't create role specific classes - that lesson has been learnt already.

I've created a sample app to demonstrate this (it includes relationships too):

1. Run "bin\debug\RolesRelationshipsSample.exe"
2. Double-click the library icons to create entities
3. Drag/drop them to assign the appropriate relationships

Feel free to play with the code. Hope you find it useful.

Answer 4

You're modeling a many-to-many relationship: a project can have many people working on it, and a person can work on multiple projects.

You're modeling the relation as a Project Role, which in addition to serving as a bi-directional link from Person <-> Project, also records a RoleType and start/end of that Person filling that RoleType on that Project. (Notice how the English work "that" stands in for the database FK or, in code, a pointer/reference?)

Because of those FKs, we can in the database follow the graph from Person, through Project Role, to Project:

select a.person_id, b.project_role_id, c.project_id
from person a join project_role b on (a.id = b.person_id)
join project c on (b.project_id = c.id)
where a.person_id = ?

Or we can follow it in the other direction, from Project:

select a.person_id, b.project_role_id, c.project_id
from person a join project_role b on (a.id = b.person_id)
join project c on (b.project_id = c.id)
where c.project_id = ?

Ideally, we'd like to be able to do the same in the C# code. So yes, we want a Person to have a list, and Project to have a list, and a ProjectRole references to a Person and a Project.

Yes, Project::addPerson( Person& ) should really be Project::addProjectRole( ProjectRole& ), unless we decide that Project::addPerson( Person& ) is a convenience method of the form:

void Project::addPerson( Person& p ) {
  this.addProjectRole( new ProjectRole( p, &this, RoleType::UNASSIGNED ) ;
}

A ProjectRole doesn't have a list, it has-a reference to a Person and a reference to a Project. It also has, as values, a start date, an end date, and a RoleType (which either is an enum, or a class instance that mimics an enum value -- that is, there is only one object per enum type, and it's stateless, immutable and idempotent, and thus sharable among many ProjectRoles).

Now this shouldn't mean that retrieving a Person from the database should cause the whole database to be reified in the object graph in the code; lazy proxies that retrieve only on use can save us from that. Then if we're only currently concerned with the Person, and not his Roles (and Projects, we can just retrieve the Person. (NHibernate, for instance, I think does this more-or-less seamlessly.)

Basically, I think that:

1) This is a standard way of representing many-to-many relations; 2) It's standard for a relation to have additional data (when, what kind of) and; 3) you've pretty much got the right idea, and are just being rightly conscientious in getting feedback here.

Answer 5

It appears that there are two main entities - Project and Project Member. The Project Member has the attributes 'Member Role' and 'Member Name'. Either of these attributes may belong to a domain ie a set of values that can be maintained in lookup tables both for convenience and to use for searching. It is assumed that someone requires information about all project members carrying out a particular role/job.

Note. Lookup tables can have entries added but would not normally have the value of an entry changed. Once a value is selected from the lookup table then it is considered a permanent fixture of the owning table - in this case the Project Member table.

I wouldn't expect to see a 'Person' entity or table in any business other than the convenience as a lookup table as in the case above. HR departments will keep a list of employees that have specific information that is required by Payroll etc. but there is nothing fundamental abut People that the business will need to know. NB Locate the business process to identify an entity - don't make it up.