Rails model with paperclip has_attached_file method

gistfile1.rb

class User < ActiveRecord::Base
  attr_accessor   :password
  attr_accessible :username, :realname, :email, :password, :password_confirmation, :encrypted_password, :avatar

  has_attached_file :avatar,
    :storage => :s3,
    :bucket => '<s3 bucket name>',
    :s3_credentials => {
      :access_key_id => ENV['S3_KEY'],
      :secret_access_key => ENV['S3_SECRET']
    }

end