sinatra + ActiveRecord rake task db:xxx の場合のみdatabase.ymlを読みに行くように設定する

db_initialize.rb

require 'active_record'
require 'pg'
require 'erb'

open(File.join(File.dirname(__FILE__), 'config', 'database.yml')) do |f|
  ActiveRecord::Base.configurations = YAML.load(ERB.new(f.read).result(binding))
  ActiveRecord::Base.establish_connection(ENV['RACK_ENV'])
end

Rakefile

task :before_db do
  require './db_initialize'
end

namespace :db do |ns|
  ns.tasks.each do |db_task|
    db_task.enhance([:before_db])
  end
end