the db:migrate task using ActiveRecord without Rails

Rakefile

require 'bundler/setup'
require 'active_record'
require 'logger'

namespace :db do
  desc "Migrate the database through scripts in db/migrate. Target specific version with VERSION=x"
  task :migrate => :environment do
    ActiveRecord::Migrator.migrate('db/migrate', ENV["VERSION"] ? ENV["VERSION"].to_i : nil )
  end

  task :environment do
    ActiveRecord::Base.logger = Logger.new(STDOUT)
    require_relative './config/db'
  end
end

ActiveSupport使ってそうなので ENV["VERSION"].try(:to_i) がいいかもですね