validator.rb

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 = validator_class.new
      validator_class.fields.each do |field|
        validator.send("#{field}=", record.send(field))
      end
      validator
    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