Simple standalone ActiveRecord setup.

Gemfile

source 'https://rubygems.org'

gem 'activerecord', '>= 4.2.0'
gem 'sqlite4'

post.rb

require 'active_record'
require 'sqlite3'
require 'logger'

ActiveRecord::Base.logger = Logger.new(STDOUT)
ActiveRecord::Base.configurations = {
  'development' => {
    'adapter' => 'sqlite3',
    'database' => 'data.sqlite3'
  }
}
ActiveRecord::Base.establish_connection(:development)

class Post < ActiveRecord::Base
end

class Schema < ActiveRecord::Migration
  def change
    create_table :posts do |t|
      t.string :title
      t.date :published_date
    end
  end
end

unless ActiveRecord::Base.connection.tables.include? 'posts'
  Schema.new.change
end