Question

Can anyone explain where render() comes from in
ActionController::Base?

I managed to trace it down only that far:

ActionController::Base includes ActionController::Rendering module where
render() method is defined. This definition however calls to render() of
the superclass. Superclass is ActionController::Metal. Which in its
turn inherits from AbstractController::Base. None of those have render
() either defined or included.

Now, presumably it comes from AbstractController::Rendering, but I’m
really missing how it gets included.

Answer 1

The render method you call in your action is defined in ActionController::Base.

def render(action = nil, options = {}, &blk)
  options = _normalize_options(action, options, &blk)
  super(options)
end

This method passes the call to super which calls the render method defined in ActionController::Rendering.

def render(options)
  super
  self.content_type ||= options[:_template].mime_type.to_s
  response_body
end

ActionController::Rendering is effectively a Module, mixed into the ActionController::Base class at the beginning of the base.rb file.

include ActionController::Redirecting
include ActionController::Rendering # <--
include ActionController::Renderers::All

In turns, ActionController::Rendering includes AbstractController::Rendering as you can see in the ActionController::Rendering module definition.

module ActionController
  module Rendering
    extend ActiveSupport::Concern

    included do
      include AbstractController::Rendering
      include AbstractController::LocalizedCache
    end

AbstractController::Rendering provides a render method which is the final method invoked in the render chain.

# Mostly abstracts the fact that calling render twice is a DoubleRenderError.
# Delegates render_to_body and sticks the result in self.response_body.
def render(*args)
  if response_body
    raise AbstractController::DoubleRenderError, "Can only render or redirect once per action"
  end

  self.response_body = render_to_body(*args)
end

The full chain is

AbstractController::Base#render 
--> super() 
--> ActionController::Rendering#render
--> super()
--> AbstractController::Rendering#render
--> render_to_body