shared_validation_class.rb

# Validate fields in different ActiveRecord objects with the same
# validation class without having to share the same field name.
#
# Field name is provided in the controller as follows:
# validates_with MyValidator, field: :field_name
#
def validate(record)                                    
 field = options[:field]                              
 if !(valid_field?(record[field]))                
   record.errors.add(field, "invalid_#{field}".to_sym)
 end                                                  
end