How to add validations to a specific instance of an active record object?

dynamic_validations.rb

class Banana < ActiveRecord::Base; end

banana = Banana.new
banana.valid? #=> true

banana.singleton_class.validates_presence_of :name
banana.valid? #=> true - why did the validation not work?  

banana.class.validates_presence_of :name
banana.valid? #=> false - as we'd expect...but now...

new_banana = Banana.new
new_banana.valid? #=> false - because the previous call soiled the Banana class with it's validation

# So how does one apply validations to the eigenclass of an ActiveRecord object?
# Or am I misunderstanding what .singleton_class is?

I would like to do the exact same thing. Any news on this?

You may be better off just using #alias_method_chain

# Extending a User instance with this decorator will add a validation that the :old_password attribute is valid
class User
  module PasswordProtection

    def self.extended(user)
      class << user
        attr_writer :old_password
        alias_method_chain :valid?, :password_protection
      end
    end

    def valid_with_password_protection?
      valid_without_password_protection?
      validate_old_password
      errors.empty?
    end

    private

    def validate_old_password
      unless self.valid_password?(@old_password)
        errors.add :old_password, "is invalid"
      end
    end
  end
end

Another solution would be using a Form Object and defining the validations depending on the context where they're used.

class class User
  before_validation :add_instance_validations

  private

    def add_instance_validations
      singleton_class.class_eval { validates :name, presence: true }
    end
end

I actually just blogged bout this, if anyone is still in need of a solution

http://dvg.github.io/2015/03/05/apply-activemodel-validations-by-policy.html

wrote an article specially answering this topic: http://www.eq8.eu/blogs/22-different-ways-how-to-do-rails-validations :)

A method that is directly built into Rails: https://github.com/rails/rails/blob/master/activemodel/lib/active_model/validations/with.rb#L115-L123

       class Person
         include ActiveModel::Validations

         validate :instance_validations, on: :create

         def instance_validations
           validates_with MyValidator, MyOtherValidator
         end
       end

A method that is directly built into Rails: https://github.com/rails/rails/blob/master/activemodel/lib/active_model/validations/with.rb#L115-L123

       class Person
         include ActiveModel::Validations

         validate :instance_validations, on: :create

         def instance_validations
           validates_with MyValidator, MyOtherValidator
         end
       end

This solution is not corresponding with the initial subdmision. What the author need is to be able to "inject" as ad-hoc a validator without affecting to the Base class.