What is a controller action?

The other half of a route

Rails routes: basics ended with get "partners" => "public/pages#partners", as: :partners, naming a destination without opening it. Here is that destination in this project:

# app/controllers/public/pages_controller.rb, this project's real file (trimmed)
module Public
  class PagesController < ApplicationController
    def partners
      @tiers = ::Partners::Tiers.purchasable
      @partners = ::Partners::ActivePartnersQuery.call.partners
      @inquiry = ::Partners::Inquiry.new
    end
  end
end

That's a controller action: an ordinary public method, partners here, in a class that inherits from ApplicationController. The route's "public/pages#partners" string, controller#action, matches it.

One detail is easy to miss: the leading :: on each constant. The class lives inside module Public, so a bare Partners::Tiers makes Ruby look for Public::Partners::Tiers first before falling back to the top level. :: skips that search and starts at the top. In a namespaced controller, copy that on purpose rather than dropping it.

Same shape as every model method already covered

The syntax is no different from Article's published? in ActiveRecord: model introduction; both are ordinary Ruby methods. The role differs. Rails MVC overview named it: a Controller coordinates. Here it asks Partners::Tiers and Partners::ActivePartnersQuery for data and decides no business rules itself.

One route, one action, called once per request

Visiting /partners runs partners once, from the top, on every request, with no state carried over. It's the one-shot execution Rails server & Puma described from the server side: one thread, one request, one pass through the method.

Key takeaways

• A controller action is a public method on a class inheriting from ApplicationController, matched by controller#action.
• Public::PagesController#partners is where Rails routes: basics' last example leads.
• An action coordinates, asking other objects for data: the Controller role from Rails MVC overview.
• Each request runs its action once, from the top, with no carried-over state.