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

Generally I try to keep controllers simple in terms of logic too. If business logic is required, it will go up to 'service layer' classes to handle it. This also saves repeating any code/logic too, which ultimately makes the whole project more maintainable if business logic was to change. I just keep models purely as entity objects.

I think the answer above sums it up nicely though, it is easy to over engineer a project based on design patterns: Go with whatever works for you and is most maintainable/efficient.

Correct? Either. They both compile, don't they?

Handy tricks are nice, why not use them were you can? That being said, as you've pointed out, you may get bloated models if you put all sorts of logic in them. Likewise, though, you can get bloated controllers when they do massive amounts in each action. There are ways to abstract elements out of either, if that's necessary too.

At the end of the day all design patterns are is guidelines. You shouldn't blindly follow any rule, just because someone else said it. Do what works for you, what you think gives clean, extensible code and hits whatever metrics you think make good code.

All that being said, for true idealistic MVC I would say that models should not have any external actions, they're data representations, nothing more. But feel free to disagree :-)

You should stop treating "model" in MVC like some class. Model is not a class or object. Model is a layer (in modern MVC, there has been some evolutions since the inception of concept). What people tend to call "models" are actually domain object (I blame Rails for this mass-stupidity).

The application logic (interaction between domain logic structures and storage abstraction) should be a part of model layer. To be more precise: it should be inside the Services.

The interaction between presentation layer (controllers, views, layouts, templates) and model layer should happen only through those services.

_{Application has no place in controllers. Controllers are structures of presentation layer, and they are responsible for handling user input. Please do not expoqbse deomain objects to it.}

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.