example_controller.rb

class ExampleController < ApplicationController

  def some_post_action
    @some_model = SomeModel.some_factory_method(params).
      validate(:make_one_particular_and_unique_thingy_go_red_and_stop_save)
    ...
  end

end

instance_validators.rb

# Instance validators patch. If model instance defines an instance_validators
# method, it will be invoked during validation
module ActiveRecord::InstanceValidators

  def valid?(*args, &block)
    output = super(*args, &block)
    instance_validators
    errors.empty? && output
  end

  # [14] pry(main)> m = Member.last
  # [15] pry(main)> def m.kme; errors.add(:base, 'kme'); end
  # [16] pry(main)> m.validate(:kme){ |m| m.errors.add(:base, 'foo')}
  # [17] pry(main)> m.valid?
  # => false
  # [21] pry(main)> m.errors.messages
  # => {:base=>["kme", "foo"]}
  def validate(*args, &block)
    args.each do |method|
      __instance_validator_callbacks << lambda{ self.send(method) }
    end
    __instance_validator_callbacks << block if block
    
    self # allow chaining
  end

  private

  def instance_validators
    __instance_validator_callbacks.each do |callback|
      args = [self][0...callback.arity]
      callback.call(*args)
    end
    __instance_validator_callbacks 
  end
  
  def __instance_validator_callbacks
    @__instance_validator_callbacks ||= []
  end

end
ActiveRecord::Base.send :include, ActiveRecord::InstanceValidators