polymorphic association question

Question from a relative newbie. I have the following models (rails -v Rails 2.3.8)

class Node < ActiveRecord::Base    belongs_to :containable, :polymorphic => true end

class Root < ActiveRecord::Base   has_one :node, :as => :containable

  validates_presence_of :owner   validates_numericality_of :owner, :only_integer => true, :message => "can only be whole number."   validates_inclusion_of :owner, :in => 21..30, :message => "can only be between 21 and 30." end

Now, from the console:

n=Node.new(:containable=>Root.new(:owner=>22), :name=>"aaabz")

=> #<Node id: nil, parent_id: nil, lft: nil, rgt: nil, containable_id: nil, containable_type: "Root", name: "aaa", permissions: "none", created_at: nil, updated_at: nil>

n.save

=> true => #<Node id: 66, parent_id: nil, lft: 7, rgt: 8, containable_id: 10, containable_type: "Root", name: "aaabz", permissions: "none", created_at: "2010-07-13 13:55:35", updated_at: "2010-07-13 13:55:35">

Root.find(10)

=> #<Root id: 10, owner: 22, created_at: "2010-07-13 13:55:35", updated_at: "2010-07-13 13:55:35">

I did not realize that both records (Node and the containable) would automatically be saved to the db when the container (Node) was saved, but it was a nice bonus. So this was all ok with me.

HOWEVER, the following also occurred:

n=Node.new(:containable=>Root.new(:owner=>1), :name=>"aaab")

=> #<Node id: nil, parent_id: nil, lft: nil, rgt: nil, containable_id: nil, containable_type: "Root", name: "aaab", permissions: "none", created_at: nil, updated_at: nil>

n.save

=> true

n

=> #<Node id: 65, parent_id: nil, lft: 5, rgt: 6, containable_id: nil, containable_type: "Root", name: "aaab", permissions: "none", created_at: "2010-07-13 13:47:19", updated_at: "2010-07-13 13:47:19">

That is, the containable record failed to validate, so it was not saved, while the container was ok and it WAS saved, leaving things in quite an unexpected condition.

I know I can code around this, but it seems quite odd. Am I missing something simple?

Thanks.