config_routes.rb

get '/services/:id', :to => 'service_types#show', :as => :service_type, :constraints => SlugConstraint.new(ServiceType, :position => 2)

lib_slug_constraint.rb

class SlugConstraint
  def initialize(klass, options = {})
    @klass = klass
    @position = options.fetch(:position, 1)
  end

  def matches?(request)
    @klass.exists?(:slug => request.path.split('/')[@position])
  end
end