handler.rb

class Handler
  def self.receive(model, action, message)
    begin
      "#{model.classify}::#{action.classify}".constantize.new(message).perform
    rescue NameError
      Rails.logger.info("No map for #{model.classify}::#{action.classify}")
      return
    end
  end
end

I use Temping for testing stuff like this, although it might not match this use case exactly.

  module Model; end
  Model::Action = Temping.create :action do
    def initialize(message)
      @message = message
    end
    def perform
      "Message: #{@message}"
    end
  end
  expect(Handler.receive "Model", "Action", "foo").to eq "Message: foo"