custom_validator.rb

class Model < ActiveRecord::Base
  validate :validate_multiples

  private

  def validate_multiples
    %w(attribute1 attribute2 attribute3).each do |attribute|
      value = send(attribute)

      next if value.blank?
      next if value % 0.5 == 0

      errors.add attribute, 'is not a multiple of 0.5'
    end
  end
end