How to map classes in NHibernate using Roles or Composition

Question

I believe this is a common question / problem but have not been able to find a good clean concise answer.

The Problem

How to map entities that appear to have an inheritance relationship:

Company
  Supplier
  Manufacturer
  Customer

However, a Supplier can be a Manufacturer.

or

Person
  Doctor
  Patient
  Employee

Where a Patient can be a Doctor or can be an Employee.

Suggestion: Use Roles

In discussions on the NHibernate forums, the response is often that this is multiple inheritance.

http://forum.hibernate.org/viewtopic.php?t=959076

They solution suggested is to use composition or to use "Roles". However, I cannot find any examples or explanation on how exactly to do that.

"Favor composition over inheritance." Remember that little goodie from class? In this instance I have to agree that you are trying multiple inheritance-- not possible in C# or Java (yet). I, personally, would encourage you to think about re-modeling so you have a Person object and a person has a one-to-many collection of Roles.

Answer 1

You probably want to consider using the Roles. So a Role will have a set of Persons. Or a Person will have a set of Roles or both. This would probably imply that there is an Association class that maps persons to roles.

Define a Person class with all properties that are common to people. Then define a Role super class and DoctorRole, PatientRole and EmployeeRole sub classes (Assuming that each role has different properties).

The Person class can have a Collection of roles defined and the Role class can have a Collection of people defined. Or it might be easier to create an Association class, lets call it PeopleRole.

This page explains how to do the mapping so that PeopleRole is a composite element. Look at the Order/Product/LineItem example. Your Person is like Order, PeopleRole is like LineItem and Role is like Product.

Answer 2

It seems to me that this is more a question around how to model a domain well, rather than an NHibernate mapping issue.

Once you've sorted out your domain modelling, I think you'll find the NHibernate mapping falls out relatively easily.

One place to look to get your head around the idea of modeling Roles is to look for "Color Modeling" - Jeff de Luca has some resources, though the idea originated with Peter Coad

The basic idea is to separate the identity of a participant from the role they play in an activity.

For example, you might have a Person object that captures the identify of a particular person.

Then, a completely separate object "Student" that captures the additional information to record the enrolment of a person as a student. Each instance of Student would have a reference to the person enrolled. A single person may be related to many "Student" records, one for each distinct enrolment.

In parallel, you could have a distinct "Tutor" object that records employment details when someone is hired to teach students in one-on-one situations. The Tutor object captures the additional details around how someone is employed as a tutor.

This structure gives you great flexibility - one person (Joe Bloggs) may just be a student, another person (Jane Doe) may just be a tutor, and a third (Chuck Norris) may be both.

Also, introducing another role (Lecturer, Marker, Administrator) becomes easier because the additions don't require changes to existing objects.

Answer 3

I came across a bit more commentary you might find relevant:

In a blog post on the Naked Objects blog a few different approaches are outlined discussing the pros and cons of each.

• Using an 'Any' association mapping
• Modelling roles as classes