API Controller is a lightweight version of ActionController::Base, created for applications that don't require all functionalities that a complete Rails controller provides, allowing you to create controllers with just the features that you need for API only applications.

An API Controller is different from a normal controller in the sense that by default it doesn't include a number of features that are usually required by browser access only: layouts and templates rendering, flash, assets, and so on. This makes the entire controller stack thinner, suitable for API applications. It doesn't mean you won't have such features if you need them: they're all available for you to include in your application, they're just not part of the default API controller stack.

Normally, ApplicationController is the only controller that inherits from ActionController::API. All other controllers in turn inherit from ApplicationController.

A sample controller could look like this:

class PostsController < ApplicationController
  def index
    posts = Post.all
    render json: posts
  end
end

Request, response, and parameters objects all work the exact same way as ActionController::Base.

Renders

The default API Controller stack includes all renderers, which means you can use render :json and brothers freely in your controllers. Keep in mind that templates are not going to be rendered, so you need to ensure your controller is calling either render or redirect_to in all actions, otherwise it will return 204 No Content.

def show
  post = Post.find(params[:id])
  render json: post
end

Redirects

Redirects are used to move from one action to another. You can use the redirect_to method in your controllers in the same way as in ActionController::Base. For example:

def create
  redirect_to root_url and return if not_authorized?
  # do stuff here
end

Adding New Behavior

In some scenarios you may want to add back some functionality provided by ActionController::Base that is not present by default in ActionController::API, for instance MimeResponds. This module gives you the respond_to method. Adding it is quite simple, you just need to include the module in a specific controller or in ApplicationController in case you want it available in your entire application:

class ApplicationController < ActionController::API
  include ActionController::MimeResponds
end

class PostsController < ApplicationController
  def index
    posts = Post.all

    respond_to do |format|
      format.json { render json: posts }
      format.xml  { render xml: posts }
    end
  end
end

Make sure to check the modules included in ActionController::Base if you want to use any other functionality that is not provided by ActionController::API out of the box.