Rails - Apr 2014 - Programmatically copying ActiveModel validators from one model to another?

I'm writing a library that will require programmatically copying 
validations from one model to another, but I'm stumped on how to pull this 
off.

I have a model that is an `ActiveModel::Model` with some validation:

    class User < ActiveRecord::Base
      validates :name, presence: true
    end

And another model that I'd like to have the same validations:

    class UserForm
      include ActiveModel::Model
      attr_accessor :name
    end

Now I'd like to give `UserForm` the same validations as `User`, and without 
modifying `User`. Copying the validators over doesn't work, because 
`ActiveModel::Validations` hooks into callbacks during the validation check:

    UserForm._validators = User._validators
    UserForm.new.valid?
    # => true             # We wanted to see `false` here, but no 
validations
                          # are actually running because the :validate 
callback
                          # is empty.

Unfortunately, there doesn't seem to be an easy way that I can see to 
programmatically give one model another's validation callbacks and still 
have it work. I think my best bet is if I can ask Rails to regenerate the 
validation callbacks based on the validators that are present at a given 
moment in time.

Is that possible? If not, is there a better way to do this?

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