custom_validator.rb

class DivisibilityValidator < ActiveModel::EachValidator
  attr_reader :base

  def initialize(options)
    super

    @base = options[:base]
    fail ArgumentError, 'You must pass the :base option!' unless base
  end

  def validate_each(record, attribute, value)
    return if value.blank? && options[:allow_blank]
    return if value.present? && value % base == 0

    record.errors.add attribute, :not_divisible, base: base
  end
end