How to represent / persist information for a table which will contain a dynamic set of configurations
create table app (id, name, desc)
create table app_config (id, app_id, key, value)

def change
  create_table :app_configs do |t|
    t.text :config
    t.integer :app_id
    t.timestamps
  end
end

class App < ActiveRecord::Base
  has_many :app_configs
  ...
end
class AppConfig < ActiveRecord::Base
  belongs_to :app
  ...
end

$ rails new testproject
$ rails generate scaffold App name:string description:string
$ rails generate scaffold AppConfig key:string value:string app:references
$ rake db:migrate

resources :apps do
  resources :app_configs
end