The +validates_exclusion_of+ helper has an option +:in+ that receives the set of values that will not be accepted for the validated attributes. The +:in+ option has an alias called +:within+ that you can use for the same purpose, if you'd like to. This example uses the +:message+ option to show how you can include the attribute's value.
The default error message for +validates_exclusion_of+ is "_is not included in the list_".
The default error message for +validates_exclusion_of+ is "_is reserved_".