usage.rb

NewPersonValidator = Validator.define do
  validates_presence_of :name
end

class Person
  attr_accessor :name
  def initialize(name); @name = name; end
end

person = Person.new("Frank")
result = NewPersonValidator.call(person)
puts "Valid? #{result.valid?}" #=> true

person = Person.new
result = NewPersonValidator.call(person)
puts "Valid? #{result.valid?}" #=> false
puts "Errors: #{result.errors.full_messages.join(', ')}" #=> "Errors: Name can't be blank"

validator.rb

module Validator
  def self.define(&block)
    klass = Class.new do
      include ActiveModel::Validations

      def initialize(target)
        @target = target
      end

      def method_missing(name, *args, &block)
        @target.send(name, *args, &block)
      end

      def self.call(object)
        validator = new(object)
        validator.valid?
        validator
      end
    end

    klass.instance_eval(&block)
    klass
  end
end