What is MVC in Ruby on Rails?

Question

Could someone please explain MVC to me in Ruby on Rails, in layman terms. I am especially interested in understanding the Model in MVC (Can't get my head around the model)

Thank you in advance.

Answer 1

I think the best way to wrap your head around MVC is by example. Try coding up a simple Rails app using MVC. There are many tutorials online, such as the blog example at "Getting Started with Rails".

If chose to learn by coding an example, check out the answers to Where can I find clear examples of MVC?

Answer 2

Your Model is the data structure that your program uses.

The View is the part that interacts with the screen or the next level up.

The Controller generally processes data between the model and view

MVC structures are often nested, so a "Model" or "View" may contain its own MVC (Think of a component on the screen. You may just fill it with a string, but behind the scenes the code of the component draws its own little view, has it's own little model (the string you pass in) and has a little controller drawing the data onto the view.

In Rails, the roles of the model, view and controller are well-defined by the framework, any tutorial will point out the three components as it walks you through the files it created.

In other systems, those pieces may be harder to identify. Also, MVC is not "Perfect", just keep in mind that there are valid alternatives, but it's a good way to start organizing.

Answer 3

MVC isn't specifically just for Ruby on Rails. It was actually created awhile before Ruby on Rails ever came around. It's mainly just a way of organizing your code so that you have some code that's responsible for your models (the Class version of a database table), some code that's responsible for your views (what's visually being displayed to the user) and code that's responsible for your controllers (what ties the views to the models and performs the bulk of your logic.

That's the non-framework-specific description. Each framework that uses MVC has a different way of implementing it. For Ruby on Rails each model represents a database table as a class that can communicate directly in code with other objects without needing to write any SQL. All the SQL is being taken care of in the background and you just have to think of it as though it were a normal class (well almost, it's not seamless yet). The view is mostly HTML and represents what will be sent to the browser. The controller is just the code that communicates the models and views together.

All in all, MVC isn't specific just to Ruby on Rails...that's just the most popular.

Answer 4

Hey Imran

Some background, MVC is a (compound) design pattern and was developed in 1979 by Trygve Reenskaug (Smalltalk).

True MVC was primarily planned for use in n-tier (non web) systems and it splits a system into 3 distinct parts, a Model, View and Controller

The Model

• Contains data for the application (often linked to a database)
• Contains state of the application (e.g. what orders a customer has)
• Contains all business logic
• Notifies the View of state changes (** not true of ROR, see below)
• No knowledge of user interfaces, so it can be reused

The View

• Generates the user interface which presents data to the user
• Passive, i.e. doesn’t do any processing
• Views work is done once the data is displayed to the user.
• Many views can access the same model for different reasons

The Controller

• Receive events from the outside world (usually through views)
• Interact with the model
• Displays the appropriate view to the user

** Classic MVC is not suited to web applications, as the model cannot send all changes to the view in an observer fashion (the view is a web page). The Model2 was introduced to overcome the changing infrastructure by JSP team in 90s . MVC Web frameworks are really not MVC, but Model2 (this is true of Ruby on Rails).

Here is a description of GUI patterns including MVC from the master, Martin Fowler GUI Architectures

The best book I have found so far is Agile Web Development with Rails. It begins by assuming no knowledge, and is quite comprehensive.

Hope this helps to shed some light for you!