Now that we understand how each individual component works in MVC, let's look at how everything works together.

The following diagram describes what happens behind the scenes in a Rails application:

Let's go through each step.

• A user makes an HTTP request

  • An HTTP request is basically a user saying, "Hey, I'm going to try to access this URL."
  • In other words, when you visit a web page, you are making an HTTP request

• The request passes through the Rails Router

  • The Rails Router identifies which controller to use
  • For example in our Ideator app, if we are trying to access the URL /ideas/1 through a GET request, the Rails Router will choose to use the IdeasController's show method, since it is defined as such in the routes.rb file:

   Prefix Verb   URI Pattern               Controller#Action
     root GET    /                         ideas#index
    ideas GET    /ideas(.:format)          ideas#index
          POST   /ideas(.:format)          ideas#create
 new_idea GET    /ideas/new(.:format)      ideas#new
edit_idea GET    /ideas/:id/edit(.:format) ideas#edit
     idea GET    /ideas/:id(.:format)      ideas#show
          PATCH  /ideas/:id(.:format)      ideas#update
          PUT    /ideas/:id(.:format)      ideas#update
          DELETE /ideas/:id(.:format)      ideas#destroy

• Notice how the GET request for the URI pattern /ideas/:id is mapped to ideas#show (the show action in the ideas controller).

• The controller receives the request

  • Often times parameters are passed into the controller. We can access these parameters using params .
  • For example, you can use params[:id] to access the id parameter

• If the controller has some code that makes a call to the model, the model receives the request to do the job

  • For example in our Ideator app, @idea = Idea.find(params[:id]) in the show method in the controller will request the model to fetch an Idea with the id of params[:id]

• The database is accessed if the model makes a request. The data is then sent back to the model.

  • For example, the Idea.find(params[:id]) code would go into the database and do the work of finding the Idea with the id of params[:id]

• The changes in the model will be sent to the controller so that it has the latest information

• The view then displays the information

• Instance variables defined in a controller method can be used in the corresponding view file

  • For example in our Ideator app, @ideas defined in the index method of ideas_controller.rb can be used in the view file, index.html.erb

• The controller then returns the response body (HTML, XML, etc.) and metadata (caching headers, redirects) to the server. The server combines the raw data into a proper HTTP response and sends it to the user.

While you build your apps, always have this flow in mind. Once you become familiar with how the application works together, you'll have an easier time debugging and writing code.