Having just finished giving three courses (2 on RavenDB and 1 on NHibernate), you might want to say that I have a lot of data stuff on my mind. Teaching is always a pleasure to me, and one of the major reasons for that is that I get to actually learn a lot whenever I teach.
In this case, in all three courses, we run into an issue with modeling associations. For the sake of the example, let us talk about employees and paychecks. You can see the model below:
Do you note the blue lines? Those represent Employee reference, but while they are both referencing the same employee, they are actually quite different associations.
The Manager association is a Current Association. It is just a pointer to the managing employee. What does this means?
Let us say that the manager of a certain employee changed her name. In that scenario, when we look at the current employee record, we should see the updated employee manager name. In this case, we are always interested in the current status.
On the other hand when looking at the paycheck PaidTo reference to an employee, we have something all together different. We have a reference no to the current employee record, but to the employee record as it was at a certain point in time. If the employee in question change his name, that paycheck was issued to Mr. Version One, not to Mr. Version Two, even though the name has been changed.
when dealing with associations, it is important to distinguish between the two options, as each require different way of working with the association.