The problem is that the MVC pattern was designed in a system that doesn't really exist anymore. It was invented in Smalltalk at a time when UI libraries did not exist. To make a window dialog you drew all the boxes, highlighted the appropriate squares, made sure that the text you were drawing ended up in the right spot...etc...
Imagine what it would be like to write a dialog app using nothing but one large canvas. That's the world the MVC comes from.
A "view" in this system was a text box and it was a class that was responsible for drawing the box, the text, drawing selected areas, responding to changes in the text, etc...
A "controller" was another class that took mouse events that occured within this box like mouse moving, key down, key up, clicks, etc...and it would decide what happened. Should we change the text? Should we change the selection? Stuff like that.
A "model" was yet another class that represented the basic data and state of the component. A text box model would have the text of course, the font, selection, etc...
As you can see, in a situation like this the three components are very entangled in the representation of a single idea. It makes sense in this context to speak of a "triad".
Today, if you're working on creating a UI library and using raw drawing commands you might do something similar. But the application of the "MVC" pattern has spread beyond its initial purpose. Now days you have a "view" that may actually be a complete dialog, and a controller that's responding to events like "textChanged" or "buttonClicked". The model in today's MVC is normally something fairly disconnected from the system (but generally linked to the view by providing an observer interface of some sort) and there may be many views associated with the one model.
In a system I recently architected for example we had around 10+ views all observing a single document "holder" and its active document. A main drawing interface interacted with the layout of the document, various property views that observed the selected item and provided a record interface, and a smaller scale representation of the main view that showed the entire document instead of just the visible window. Some of these views had controllers of varying complexity that turned GUI events into changes to the document, which would in turn notify its various views.
Can you still call such a relationship a "triad"? Perhaps, but I think it implies too much of the former, older application of MVC.
Could you share controllers with different views? Depends on how similar the views are. I've found that generally speaking this type of object has behavior to specific to the view it's controlling AND the model it is manipulating to be very reusable...but there's always exceptions.
The ViewModel object is not what gets stored in a database table, generally. It's the individual items in the ViewModel object that get stored. Each of those items already has an ID.
For example:
public class InvoiceViewModel
{
public Customer Customer { get; set; }
public Address BillingAddress { get; set; }
public Address ShippingAddress { get; set; }
public List<InvoiceLineItem> Items { get; set; }
}
Since there's no single table in the database that corresponds to an InvoiceViewModel, there is no ID for an InvoiceViewModel object.
Of course, you can always use the InvoiceID as the id for this particular ViewModel. InvoiceID is handy, because that's what this object ultimately represents. But I could see having a ViewModel object that doesn't correspond to any particular ID in the database.
Best Answer
It depends on the type of MVC you want. I suggest you try the Michael Feather's "humble dialog box", which is also called "model view presenter (MVP)", and probably what you are looking for.
In this variant, the view has a refererence to the controller, and the controller has a reference to an abstract interface of the view. So the view can provide methods for registering listeners, those methods are also available in the interface, and the controller will call these methods without having direct access to the UI.