class String

def underscore

See ActiveSupport::Inflector.underscore.

'ActiveModel::Errors'.underscore # => "active_model/errors"
'ActiveModel'.underscore # => "active_model"

+underscore+ will also change '::' to '/' to convert namespaces to paths.

The reverse of +camelize+. Makes an underscored, lowercase form from the expression in the string. 
def underscore
  ActiveSupport::Inflector.underscore(self)
end