Assigning default values to attributes in a has_one association

Hi List,

I've a little problem here, for which I can't find a solution:

I have two models, person and event. Event is a polymorphic association (timeable). In my person model I would like to define an association with Events:

class Person < ActiveRecord::Base

    has_one birthday,         :dependent => :destroy,         :as => :timeable,         :class_name => "Event",         :foreign_key => :timeable_id,         :conditions => "category = 'birthday'"

end

As you can see, there's a category field in the event table, which should contain the category of the Event. In this case, is should contain "birthday".

I would like this category attribute to get set automatically to "birthday" whenever I create a new association.

So, calling simething like

@client.build_birthday(params[:birthday])

in my controller should set the category attribute to "birthday". I was trying something like:

    has_one birthday,         :dependent => :destroy,         :as => :timeable,         :class_name => "Event",         :foreign_key => :timeable_id,         :conditions => ['category = ?', name.to_s] do             self.category = "birthday"         end

But this seems not to work.

Also there are callbacks for collection associations (i.e. before_add and after_add), but this does not work with has_one associations.

So, what would be the best way of doing womthing like this (I sure there is at least _one_ way...)

Regards, Timo