Ruby on Rails

Hi,
I'm developing a app with dynamic field using hstore ( postgres ).
The app works great, but now, i'm trying to implement validations.
my code is:

#model
..
def initialize_custom_fields
  class_eval do
    validates field.f, :presence => true
  end
end

..

#user controller
def create
  user = User.new
  user.initialize_custom_fields
  user.attributes = params[:user]
  user.save
end

The first time I press "save" button, the validations works as well, but
the second one i'm getting this error:

undefined local variable or method `_callback_before_170045' for
#<User:0x007f80f9990558>

Anyone have a idea for solution this error.. ?