execly_one_validator.rb

class ExeclyOneValidator < ActiveModel::Validator
  def validate(record)
    raise ArgumentError, ":fields option doesn't supplied" unless options.has_key?(:fields)
    if options[:fields].select { |f| record.read_attribute(f.to_s).present? }.size != 1
      _fields = options[:fields].map(&:to_s).map(&:titleize).join(", ")
      record.errors[:base] << "execly one of #{_fields} must be present" 
    end
  end  
end