u=URI.parse(ENV['MONGOHQ_URL'])

DataMapper.setup(:external,
      :adapter => 'mongo',
      :user => u.user,
      :password => u.password,
      :host => u.host,
      :port => u.port,
      :database => u.path.gsub(/^\//, '')
)

but now i got an error talking with the database:

unauthorized db:app1234567 lock type:-1 client:12.345.67.891