Custom validation objects

parent_registration_validator.rb

class ParentRegistartionValidator < ActiveModel::Validator
  def validate(record)
    record.errors.add(:weight, :blank) unless record.weight.present?
    ...
  end
end

registration.rb

class Registration
  validates_with ParentRegistartionValidator, :if => proc { |r| r.validator == "parent_registration" }
  attr_accessor :validator

  ...
end

registration_wizard_controller.rb

class RegistrationWizardController
  def update
    @registration = Registration.find(params[:id])
    @registration.validator = "parent_registration"
    if @registration.update_attribute(params[:registration])
      redirect_to(url)
    else
      render(:edit)
    end
  end

  ...
end