Should be only:

class Ticket < ActiveRecord::Base
  default_scope -> { order(:external_updated_at) } 
end

default_scope accept a block, lambda is necessary for scope(), because there are 2 parameters, name and block:

class Shirt < ActiveRecord::Base
  scope :red, -> { where(color: 'red') }
end

This is what worked for me:

default_scope  { order(:created_at => :desc) }