How should my MVC Model work with my Business logic classes?

Question

So I have a MVC app and in another project I have a normal collection of classes which handle the Business and Data logic for the application. I also have some logic in the Model of the MVC project itself. This logic handles ViewModels and the like, things which could not have been done in the n-tier project as they relate to the MVC project itself and need to be in the same project.

My questions are:

• Should my model classes have knowledge of the n-tier business logic? Or should only the controller have this knowledge and send data back and forth between the n-tier application and the MVC model as needed?
• If it's ok for my model to reference the n-tier application, then should my controller access n-tier via the model class?

Hope this makes sense, found it difficult to word correctly to get my point across.

Answer 1

Think of your models as only containers for data between your controllers and your views. Essentially DTOs.

Answer 2

Generally speaking here - Your model classes should not have business logic knowledge. They should have only the information required to display the view to the user (use DTOs as suggested by mxmissile).

Your business logic would either be in your controller, or (better) in a separate service layer called by your controller. Having methods on a model that, for instance, bypass the controller and make calls directly to the database is almost always a bad practice.

The idea here is to make the views as dumb as possible. You send them a model, they pull out the data they need, format it appropriately, and display it. This makes it much easier to create new views of the same data later if you decide you want to change the presentation.

Answer 3

Your ViewData/ViewModel classes in your MVC application may contain instances of your Model classes (mine do). My controllers call my business services and are responsible for any translation between ViewData and Models.

If it's ok for my model to reference the n-tier application, then should my controller access n-tier via the model class?

I wouldn't go through the model to get to the application tiers, I would have the controller be that interfacing component. The controller calls to your application tiers which return instances of your Model from your data access components. You can then translate those instances into more consumable objects by using a ViewData/ViewModel object. You can do this in a controller, or use a separate assembler class.