Circular dependency and object creation when attempting DDD
- by Matthew
I have a domain where an Organization has People.
Organization Entity
public class Organization {
private readonly List<Person> _people = new List<Person>();
public Person CreatePerson(string name) {
var person = new Person(organization, name);
_people.Add(person);
return person;
}
public IEnumerable<Person> People {
get { return _people; }
}
}
Person Entity
public class Person
{
public Person(Organization organization, string name) {
if (organization == null) {
throw new ArgumentNullException("organization");
}
Organization = organization;
Name = name;
}
public Organization { get; private set; }
public Name { get; private set; }
}
The rule for this relationship is that a Person must belong to exactly one Organization.
The invariants I want to guarantee are:
A person must have an organization
this is enforced via the Person's constuctor
An organization must know of its people
this is why the Organization has a CreatePerson method
A person must belong to only one organization
this is why the organization's people list is not publicly mutable (ignoring the casting to List, maybe ToEnumerable can enforce that, not too concerned about it though)
What I want out of this is that if a person is created, that the organization knows about its creation.
However, the problem with the model currently is that you are able to create a person without ever adding it to the organizations collection.
Here's a failing unit-test to describe my problem
[Test]
public void AnOrganizationMustKnowOfItsPeople()
{
var organization = new Organization();
var person = new Person(organization, "Steve McQueen");
CollectionAssert.Contains(organization.People, person);
}
What is the most idiomatic way to enforce the invariants and the circular relationship?
