When an HTTP request arrives from the browser it needs to be matched to a controller action, depending on the HTTP verb (GET, POST, PUT, DELETE). If there’s no match it will throw an error.

It grabs the parameters and makes them available as params.

The routes are defined in config/routes.rb, and you can list them in the command line running rails routes.

Root

The most important, defined as:

root to: "kittens#index" # kittens controller, index action

RESTful routes

There are seven types of action that you can use with a “resource”, like a blog post or user. Something with a model.

1. GET all the posts (index).
2. GET a specific post (show).
3. GET the page that lets you create a new post (new).
4. POST the data from the new page so the server can create the new post (create).
5. GET the page to edit a post (edit).
6. POST the data from edit to update the post info (update).
7. DELETE a post using a delete request (destroy).

The long way to define these routes:

get "/posts", to: "posts#index"
get "/posts/new", to: "posts#new"
get "/posts/:id", to: "posts#show"
post "/posts", to: "posts#create"  # usually a submitted form
get "/posts/:id/edit", to: "posts#edit"
put "/posts/:id", to: "posts#update" # usually a submitted form
delete "/posts/:id", to: "posts#destroy"

The show way to define all seven routes:

resources :posts

# Or, if you don't want all routes
resources :posts, only: [:index, :show]
resources :users, except: [:index]

You can define non-RESTful urls as well:

get '/somepath', to: 'some_controller#some_action'

Again, running rails routes you will see all the routes available. For example:

edit_post  GET  /posts/:id/edit(.:format)  posts#edit

The (.:format) part means that it’s optional to include the format (which later would be included in the params).

The edit_post part is the route name. Rails provide helpers to generate the partial url appending _path (as in edit_post_path) or to generate the full url with _url.