underscore

Makes an underscored, lowercase form from the expression in the string.

Changes '::' to '/' to convert namespaces to paths.

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

As a rule of thumb you can think of underscore as the inverse of camelize, though there are cases where that does not hold:

'SSLError'.underscore.camelize # => "SslError"