add

Adds a new renderer to call within controller actions. A renderer is invoked by passing its name as an option to AbstractController::Rendering#render. To create a renderer pass it a name and a block. The block takes two arguments, the first is the value paired with its key and the second is the remaining hash of options passed to render.

Create a csv renderer:

ActionController::Renderers.add :csv do |obj, options|
  filename = options[:filename] || 'data'
  str = obj.respond_to?(:to_csv) ? obj.to_csv : obj.to_s
  send_data str, type: Mime::CSV,
    disposition: "attachment; filename=#{filename}.csv"
end

Note that we used Mime::CSV for the csv mime type as it comes with Rails. For a custom renderer, you'll need to register a mime type with Mime::Type.register.

To use the csv renderer in a controller action:

def show
  @csvable = Csvable.find(params[:id])
  respond_to do |format|
    format.html
    format.csv { render csv: @csvable, filename: @csvable.name }
  }
end

To use renderers and their mime types in more concise ways, see ActionController::MimeResponds::ClassMethods.respond_to and ActionController::MimeResponds#respond_with