cron.rake

task :cron => :environment do
  DatabaseBackup.back_up_to_s3!
end

database_backup.rb

require 'open-uri'
require 'heroku'
require 'heroku/command'
require 'heroku/command/auth'
require 'heroku/command/pgbackups'

class DatabaseBackup
  def self.back_up_to_s3!
    app_name = ENV.fetch('HEROKU_APP_NAME')
    Heroku::Command.run 'pgbackups:capture', ['--expire', '--app', app_name]
    url = capture_stdout do
      Heroku::Command.run 'pgbackups:url', ['--app', app_name]
    end

    AWS::S3::Base.establish_connection!(
      :access_key_id => ENV.fetch('AMAZON_ACCESS_KEY_ID'),
      :secret_access_key => ENV.fetch('AMAZON_SECRET_ACCESS_KEY'))
    url.gsub! /^"|"$/, ''
    AWS::S3::S3Object.store(
      "pgdb-#{Time.now.strftime('%y%m%d_%H%M')}.dump",
      open(url),
      ENV.fetch('S3_BACKUP_BUCKET'))
  end

  def self.capture_stdout
    out = StringIO.new
    $stdout = out
    yield
    return out.string
  ensure
    $stdout = STDOUT
  end
end

class Heroku::Auth
  def self.client
    Heroku::Client.new ENV.fetch('HEROKU_LOGIN'), ENV.fetch('HEROKU_PASSWORD')
  end
end

The AWS gem is required by Bundler. I only explicitly require the Heroku gem because I had to for some reason, even though it was in my Gemfile. It seems that requiring 'heroku' doesn't also require those other bits.