Rails Controllers - Request Processing

In the last lecture we say how the dispatcher routes a request to a particular controller action (method).

The HTTP GET request
will route to the show method in the PostsController class, passing params[:id] with a value of 1 to the controller. Note: this class is defined in the file: ./app/contollers/posts_controller.rb.

Next, the show method will use the ActiveRecord#find method to retrieve the post with id=1 from the database, and assign it to the instance variable @post.

Finally, the controller will pass @post to the view, i.e., to the template file:./app/views/posts/show.html.erband this will be used to create the HTML that will be sent to the browser.


The PostsController#show method is defined as follows:

# GET /posts/1
# GET /posts/1.json
def show
It doesn’t retrieve the post!

The desired post is actually retrieved from the database using a filter called set_post.

Filters allow controllers to run shared pre and post processing code over their methods.


In general, the “state” of an application which needs to persist across requests should be stored in the database. E.g., posts and comments are persisted in the database.

There are times when data needs to be persisted differently. E.g., the current contents of a shopping cart.

Whenever a user connects to a Rails application, a session is created.

Session data is stored in Rails using a hash structure that persists across requests, and can be accessed by controllers. Ex. session[:current_user] = user.id

A flash hash is part of the session that is cleared with each request (its value is made to the next request). A controller can use this to send a message that can be displayed to the user on the next request.

lstinlineflash[: notice] =0 Postwassuccessfullycreated.

Response - Rendering HTML or JSON

The request:


assumes that HTML will be returned. I.e., it’s the same as:


Rails can return other formats, e.g., JSON capabilities are also provided by default.
Ex. The following request, will be routed to the same controller method as before:


However, it will be rendered using the file:

and JSON will be returned to the client.

Response Format

One last look at rake routes:

Prefix      Verb      URI Pattern                   Controller#Action
posts       GET       /posts(.:format)              posts#index
            POST      /posts(.:format)              posts#create
new_post    GET       /posts/new(.:format)          posts#new
edit_post   GET       /posts/:id/edit(.:format)     posts#edit
post        GET       /posts/:id(.:format)          posts#show
            PATCH     /posts/:id(.:format)          posts#update
            PUT       /posts/:id(.:format)          posts#update
            DELETE    /posts/:id(.:format)          posts#destroy


Rails may also respond to an HTTP request using the redirect_to method.

This method actually tells the browser to send a new request for a different URL.

redirect_to ’www.example.com’

Rails has shortcuts for URLs within your application – they’re the prefix listed when you view routes:

redirect_to posts_url
will redirect to the index method in the PostsController.
You can assign a flash message as a part of a redirection:
redirect_to @post, notice: ’Post was successfully created.’