Polymorphic association and saving a has_one association

Hi,

I've got a little problem with a polymorphic association and has_one.

Here's my current setup:

class Client
   has_one :birthday, :dependent => :destroy, :as => :timeable, :class_name
=> "Event", :foreign_key => :timeable_id
end

class Event
   belongs_to :timeable, :polymorphic => true
end

Now I try to edit an existing client and try to save the associated event:

def update
  @client = Client.find(params[:id])
  @client.build_birthday(params[:birthday])
  ...
end

Saving this @client Objekt created a new entry in the Event Databasetable. I
thought, it would overwrite the existing entry in the Event-table, as it's
a "has_one" association and not a "has_many".

Of course I could write something like (this is working!):

def update
  @client = Client.find(params[:id])
  @client.birthday = Event.new(params[:birthday])
  ...
end

but I would like to avoid the word "Event" in the update method, as it's the
only appearance of the name of the class, as I try to put the above stuff
including the Event class into a plugin.

So, why does @client.build_birthday adds a new object and what's the correct
way of assigning an instance object to a has_one association?

Regards,
Timo