Simple rails example of form interacting with non-activerecord backend code

application.rb

# config/application.rb

require File.expand_path('../boot', __FILE__)

# Pick the frameworks you want:
require "active_model/railtie"
# require "active_record/railtie"
require "action_controller/railtie"
require "action_mailer/railtie"
require "action_view/railtie"
# require "sprockets/railtie"
# require "rails/test_unit/railtie"

# Require the gems listed in Gemfile, including any gems
# you've limited to :test, :development, or :production.
Bundler.require(*Rails.groups)

module Foo
  class Application < Rails::Application
    # Settings in config/environments/* take precedence over those specified here.
    # Application configuration should go into files in config/initializers
    # -- all .rb files in that directory are automatically loaded.

    # Set Time.zone default to the specified zone and make Active Record auto-convert to this zone.
    # Run "rake -D time" for a list of tasks for finding time zone names. Default is UTC.
    # config.time_zone = 'Central Time (US & Canada)'

    # The default locale is :en and all translations from config/locales/*.rb,yml are auto loaded.
    # config.i18n.load_path += Dir[Rails.root.join('my', 'locales', '*.{rb,yml}').to_s]
    # config.i18n.default_locale = :de

    # Let Rails autoload from lib directory
    config.autoload_paths << Rails.root.join('lib') # <------- ADDED THIS LINE
  end
end

foo_controller.rb

# controllers/foo_controller.rb

class FooController < ApplicationController
  def index
    @something = Widget.something
  end

  def bar
    answer = params[:foo][:something].to_i
    flash[:info] = "Your answer (#{answer}) was #{Widget.check answer}!"
    redirect_to action: :index
  end
end

index.html.erb

<!-- app/views/foo/index.html.erb -->

<% if flash[:info] %>
  <span class="flash success"><%= flash[:info] %></span>
<% end %>
<%= form_for :foo, url: foo_bar_url do |f| %>
  <p>Here's a value from the widget:</p>
  <%= f.text_field :something, value: @something %></p>
  <%= f.submit "Bar!" %>
<% end %>

routes.rb

# config/routes.rb

Rails.application.routes.draw do
  get "foo" => "foo#index"
  post "foo/bar" => "foo#bar"
end

widget.rb

# lib/widget.rb

class Widget
  def self.something
    Random.rand 100
  end

  def self.check number
    number == 42 ? "correct" : "wrong"
  end
end