Yes or no: Should models in MVC contain application logic?

Question

Yesterday I had some discussion with one of our developers regarding MVC, more precisely about the role of the model component in MVC.

In my opinion, a model should

Answer 1

Your suggestion about the modules (without any business logic inside) sounds more like you talk about Value Objects. The suggestion of your college sounds more like Domain Objects.

In my opinion the concept which will be used depends on the framework which is used (that's the practical view, the more philosophical one is bellow). If framework is used it usually sets rules about how you should implement each component. For example we can look at different MVC frameworks. In Flex's Cairngorm framework we have both. VO (Value objects) are primary used for bindings to the view, while the DO (Domain objects) hold the business logic. If we look at ASP.NET's MVC implementation there we have a model which contains at least the data (VO) but also some validation (if required). Let us look at a UI MV* framework - for example Backbone.js. Backbone's documentation says:

Models are the heart of any JavaScript application, containing the interactive data as well as a large part of the logic surrounding it: conversions, validations, computed properties, and access control.

If we look into the traditional MVC provided by Smalltalk we see that: "Model: manages the behavior and data of the application domain" so we have some behavior in it, not just plain data.

Let's think practically, if don't have any logic in the Model probably we should put all the application and business logic into the Controller.

Now let's focus on a concrete example. Imagine we have a model which is a Graph. We want to find the shortest path between two nodes in it. A good question is where to put the algorithm which finds the shortest path? It's a kind of business logic, right? If we look at the main benefits of MVC (code reuse, DRY etc.) we can see that if we want to reuse our model in the best possible way we should implement the shortest path inside it. The shortest path algorithm usually depends on the graph inner representation (or at least for best performance of the algorithm) but this representation is encapsulated into the model, unfortunately we cant reuse the full shortest path for matrix representation and list of neighbors so it's not a good idea to put it into the controller.

So as conclusion I can said that it depends on your needs (mostly). The traditional MVC purpose is to be used in the UI (inside GoF

The Model/View/Controller (MVC) triad of classes [first described by Krasner and Pope in >1988] is used to build user interfaces in Smalltalk-80.

)

now we use it in different areas - only UI, for web applications, etc. It cant be used in it's pure form because of that. But anyway, in my opinion the best separation of concerns can be achieved by isolation of the business logic into the Model and the application logic into the Controller.