How do you handle objects that need custom behavior, and need to exist as an entity in the database?
- by Scott Whitlock
For a simple example, assume your application sends out notifications to users when various events happen. So in the database I might have the following tables:
TABLE Event
EventId uniqueidentifier
EventName varchar
TABLE User
UserId uniqueidentifier
Name varchar
TABLE EventSubscription
EventUserId
EventId
UserId
The events themselves are generated by the program. So there are hard-coded points in the application where an event instance is generated, and it needs to notify all the subscribed users.
So, the application itself doesn't edit the Event table, except during initial installation, and during an update where a new Event might be created.
At some point, when an event is generated, the application needs to lookup the Event and get a list of Users. What's the best way to link the event in the source code to the event in the database?
Option 1:
Store the EventName in the program as a fixed constant, and look it up by name.
Option 2:
Store the EventId in the program as a static Guid, and look it up by ID.
Extra Credit
In other similar circumstances I may want to include custom behavior with the event type. That is, I'll want subclasses of my Event entity class with different behaviors, and when I lookup an event, I want it to return an instance of my subclass.
For instance:
class Event
{
public Guid Id { get; }
public Guid EventName { get; }
public ReadOnlyCollection<EventSubscription> EventSubscriptions { get; }
public void NotifySubscribers()
{
foreach(var eventSubscription in EventSubscriptions)
{
eventSubscription.Notify();
}
this.OnSubscribersNotified();
}
public virtual void OnSubscribersNotified() {}
}
class WakingEvent : Event
{
private readonly IWaker waker;
public WakingEvent(IWaker waker)
{
if(waker == null) throw new ArgumentNullException("waker");
this.waker = waker;
}
public override void OnSubscribersNotified()
{
this.waker.Wake();
base.OnSubscribersNotified();
}
}
So, that means I need to map WakingEvent to whatever key I'm using to look it up in the database. Let's say that's the EventId. Where do I store this relationship? Does it go in the event repository class? Should the WakingEvent know declare its own ID in a static member or method?
...and then, is this all backwards? If all events have a subclass, then instead of retrieving events by ID, should I be asking my repository for the WakingEvent like this:
public T GetEvent<T>() where T : Event
{
...
// what goes here?
...
}
I can't be the first one to tackle this. What's the best practice?
