default value for attribute in model

If you want to run some code after the record was found in DB, take a look at
after_find and after_initialize callbacks.

class MyClass
  def initialize (params=nil)
    super(params)
    @allow_validation = true
  end

  def after_initialize
    @allow_validation = true
  end
end

But I would recommend to use some other method to avoid validation.

Take a look at Acts As Authenticated plugin and Login Engine. Those should
contain ready solutions for that problem.