Dragonfly and MongoMapper and Heroku

dragonfly.rb

require 'dragonfly'
require 'dragonfly/mongo_data_store'

require 'mongo'
require 'uri'

# From https://devcenter.heroku.com/articles/mongohq#use-with-ruby
def get_connection
  return @db_connection if @db_connection
  db = URI.parse(ENV['MONGOHQ_URL'])
  db_name = db.path.gsub(/^\//, '')
  @db_connection = Mongo::Connection.new(db.host, db.port).db(db_name)
  @db_connection.authenticate(db.user, db.password) unless (db.user.nil? || db.user.nil?)
  @db_connection
end

# Configure
Dragonfly.app.configure do
  plugin :imagemagick

  protect_from_dos_attacks true
  secret "cbbbf...3ab879"

  url_format "/media/:job/:name"

  # Added this bit to tweak the dragonfly mongo options to point to MongoHQ
  if ENV['MONGOHQ_URL']
    uri = URI.parse(ENV['MONGOHQ_URL'])
    name = uri.path.gsub(/^\//, '')

    datastore :mongo,
              db: get_connection,
              root_path: Rails.root.join('public/system/dragonfly', Rails.env),
              server_root: Rails.root.join('public')
  else
    datastore :mongo,
              db: MongoMapper.database,
              root_path: Rails.root.join('public/system/dragonfly', Rails.env),
              server_root: Rails.root.join('public')
  end

end

# Logger
Dragonfly.logger = Rails.logger

# Mount as middleware
Rails.application.middleware.use Dragonfly::Middleware

# Add model functionality
if defined?(ActiveRecord::Base)
  ActiveRecord::Base.extend Dragonfly::Model
  ActiveRecord::Base.extend Dragonfly::Model::Validations
end

See also a mongoid version: https://gist.github.com/rdetert/5538121