validators.rb

class PersonValidator < SimpleDelegator
  include ActiveModel::Validations
  validates :name, presence: true
end

class Validator < ActiveModel::Validator
  class Factory
    @validators = {}

    def self.register(klass, validator)
      @validators[klass] = validator
    end

    def self.build(record)
      validator_class = @validators[record.class]
      unless validator_class
        fail "Validator not registered for #{record.class}"
      end

      validator_class.new(record)
    end
  end

  def validate(record)
    validator = Factory.build(record)
    validator.valid?

    validator.errors.each do |key, error|
      record.errors.add(key, error)
    end
  end
end

class Person
  include ActiveModel::Validations
  Validator::Factory.register(self, PersonValidator)
  validates_with Validator

  attr_accessor :name
end