A validator that checks the presence of other related fields that should be present. It would probably always be used in conjunction with :allow_blank => true.

dependent_presence_validator.rb

class DependentPresenceValidator < ActiveModel::EachValidator

  def validate_each(record, attribute, value)

    dependents = [options[:dependents]].flatten

    dependents.each do |dependent|
      unless record.send(dependent).present?
        record.errors.add dependent, "must not be blank when #{attribute.to_s.humanize} is present."
      end
    end

  end

end

person.rb

# example usage
class Person

  validates :name,
    :dependent_presence => { :dependents => [:dob, :phone] },
    :allow_blank => true

end