Ruby on Rails. Key value table for site configuration integrated with Active Admin and Formtastic forms. Updated to use erb instead of haml.

20120118012543_create_site_configuration.rb

# db/migrations/20120118012543_create_site_configuration.rb

class CreateSiteConfigurations < ActiveRecord::Migration
  def change
    create_table :site_configurations do |t|
      t.string  :key
      t.text    :value
      t.string  :form_type
      t.string  :form_collection_command

      t.timestamps
    end

    add_index :site_configurations, :key, unique: true
  end
end

index.html.erb

<%= semantic_form_for :update_site_configuration, :url => admin_site_configuration_path(0), :method => :put do |form| %>
  <%= form.inputs do %>
    <% SiteConfiguration.all.each do |record| %>
      <%= form.input record.key.to_sym, as: record.form_type.to_sym, input_html: {value: record.value}, :include_blank => false, collection: (options_from_collection_for_select(eval(record.form_collection_command), 'id', 'to_s', record.value) if record.form_collection_command.present?) %>
    <% end %>
  <% end %>
  <%= form.actions do %>
    <%= form.action :submit, label: "Update Site Configuration" %>
  <% end %>
<% end %>

site_configuration.rb

# app/models/site_configuration.rb

class SiteConfiguration < ActiveRecord::Base
  serialize :value

  class_attribute :settings
  self.settings = []

  def self.ensure_created
    self.settings.each do |setting|
      self.send(setting)
    end
  end

  def self.setting(name, default, form_type, form_collection_command="")
    class_eval <<-EOC
      self.settings << "#{name}"
      def self.#{name}
        @#{name} ||= self.find_or_create_by_key("#{name}", value: #{default}, form_type: "#{form_type}", form_collection_command: "#{form_collection_command}").value
      end
      def self.#{name}=(value)
        record = self.find_or_create_by_key("#{name}", value: #{default}, form_type: "#{form_type}", form_collection_command: "#{form_collection_command}")
        record.value = value
        record.save!
        @#{name} = value
      end
    EOC
  end

  # Define settings by listing them here
  setting :contact_page_id,           2,                      :select,  "Page.all"
  setting :top_menu_id,               1,                      :select,  "Menu.all"
  setting :customer_service_menu_id,  2,                      :select,  "Menu.all"
  setting :footer_menu_id,            3,                      :select,  "Menu.all"
  setting :notification_addresses,    "'admin@example.com'",  :string

  # Ensure all the defaults are created when the class file is read. Note that unless the migration has run
  # this will break 'rake db:migrate', so comment this out and run the migration first.
  self.ensure_created
end

site_configurations.rb

# app/admin/site_configurations.rb

ActiveAdmin.register SiteConfiguration do
  actions :only => [:index, :update]

  before_filter do
    @skip_sidebar = true
  end

  config.comments = false
  config.clear_action_items!

  controller do
    def index
      params[:action] = "Site Configuration" # for the active admin title
      render 'admin/settings/index', :layout => 'active_admin'
    end
    def update
      params[:update_site_configuration].each do |setting, value|
        SiteConfiguration.send("#{setting}=", value)
      end
      redirect_to :back, notice: "Settings Saved"
    end
  end

end

Nice!