CSC/ECE 517 Fall 2012/ch1b 1w64 nn
SaaS - 3.12 Controller and views
This article focuses on the concepts and usage of controllers and views in Rails application which is available as one of the SaaS video lectures. It gives an overview of MVC responsibilities in a Rails application with the main focus on the creation of controllers and views for a specific model.
In Rails, applications are broken into three types of components: models, views, and controllers.
- Model: Application's state is maintained by models. It not only stores the state of the application in databases but also imposes business rules on that data. It contains methods to get and manipulate data. Different methods are provided by ActiveRecord to do that . An example is shown below.
class Movie < ActiveRecord::Base belongs_to :genre has_many :actor def find_movie_name(movie_id) if movie_id then movie = Movie.find_by_id(movie_id) return movie.name end end
- Controller: In REST applications, the controller acts as a middle man to synchronize the communication between model and view. It is responsible for receiving request from the user, getting data from the model and making it available to the view for displaying data to the user. Controllers are also responsible for routing the requests to internal actions. This is facilitated by helper methods . An example is shown below.
class MoviesController < ApplicationController def show @movie = Movie.find(params[:id]) end end
Here, params is the parameter from the request that is parsed out by the routing subsystem. This contains the id of the movie that the user wants to find which can be fetched by calling Movie.find. In a real application, this is unsafe and must be handled by an exception since params[:id] can happen to be a non-existing value and will result in an error. Finally the instance variable @movie in the controller is going to be available to the view.
- View: Views are responsible for creating response in the browser to display data and allow user interaction . In the above example, show is going to display the details of a movie (genre, actors). To select the view for the rendering step, the default action the rails will take is, to find a view whose directory name matches the name of its class which is movies and finally matches the name of that particular action i.e show. Thus the view for the above action can be found in app/view/movies/show.html.erb. Here ERb ( Embedded Ruby) is used to generate dynamic content in Rails application.
Adding a new controller action to Rails application
In Rails controller forms the logical center of the application. Controller is essentially a Ruby class is inherited from ActionController super class . To create a controller, these steps must be followed.
- Add the action method in the appropriate app/controllers/*_controller.rb. This method will have the actual code to do whatever the action is meant to do. For example,
class MoviesController < ApplicationController def new end end
Here the new function creates an instance of Movies controller.
- Create a route in config/routes.rb. When the application receives a request from the user, it is this route that determines the appropriate controller and action to run. Example of a route in routes.rb
match 'movies/new' => 'movies#new'
- Ensure there is something for the action to render in app/views/model/action.html.erb. Every trip through the controller has to end with returning something to the view. Even if there is no explicit render at the end of controller, by default, Rails looks for that particular action in the app/view/model. For example, the controller shown below has an index method as follows:
class MoviesController < ApplicationController def index @movies = Movie.all end end
The view for this controller will be found in app/view/movie/index.html.erb. It is shown below as follows:
<h1>Listing Movies</h1> <table> <tr> <th>Title</th> <th>Description</th> <th></th> <th></th> <th></th> </tr> <% @movies.each do |movie| %> <tr> <td><%= movie.title %></td> <td><%= movie.description %></td> <td><%= link_to 'Show', movie_path(movie) %></td> <td><%= link_to 'Edit', edit_movie_path(movie) %></td> <td><%= link_to 'Remove', movie, :confirm => 'Are you sure?', :method => :delete %></td> </tr> <% end %> </table> <br /> <%= link_to 'New movie', new_movie_path %>
The routing subsystem that maps the URI to different controller actions also provides helper methods that generates routes in the context of that page. These routes generated by helper classes is used by the user in the views to navigate from one page to another . In the above mentioned example of a view, there are three helper methods used namely, movie_path(movie), edit_movie_path(movie), new_movie_path. More examples are shown below in the table.
|Helper method||URI returned||RESTful route||Action|
The following example illustrates how the helper methods work. Lets consider that the user is looking at the list of all movies index.html.erb. Somewhere on that page there will be a link whose argument is movie_path with an argument.
movie_path is the helper that sets up and gives back a route to the show action for that movie id. So when the user clicks on it, the URI that is going to be generated is going to be movie/:id with a GET method because its a link. This is how it looks in the actual html.
From here, once the URI is hit, the controller looks up for this URI in the routing subsystem. This corresponds to the Restful route GET /movies/:id which matches the show action of the movies controller. Further it takes :id and stores as params[:id]. With that params[:id], appropriate controller action is called.
Further the user can return to the list of movies with the help of another URI helper, movies_path. movies_path with no arguments links to the index action. The line to add on show template is
link_to 'Back to List', movies_path
Here Back to List is the text that is click-able and movies_path is a method call.
Implementing MVC architecture in Rails application has few advantages. For example it separates models from presentation which in turn helps in implementing better test cases and better UI's for the user . It also separates controller functionality with views which facilitates development of better web interfaces. Also Rails provides an easy way to implement MVC architecture for a user. All these advantages and the fact that MVC can be implemented in an effortless and uncomplicated manner necessitate the use of MVC architecture in Rails application.