nestedattributes for has_one and has_many association has different saving sequence

The models schema and its association is like this :-

ActiveRecord::Schema.define(:version => 20100219043257) do
  create_table "policies", :force => true do |t|
    t.string   "policy_no"
    t.datetime "created_at"
    t.datetime "updated_at"
  end

  create_table "policy_items", :force => true do |t|
    t.integer  "policy_id"
    t.string   "item_type"
    t.decimal  "premium"
    t.datetime "created_at"
    t.datetime "updated_at"
  end
end

class Policy < ActiveRecord::Base
  has_one :basic_plan, :class_name => "PolicyItem"
  has_many :topups, :class_name => "PolicyItem"

  accepts_nested_attributes_for :basic_plan, :allow_destroy => true
  accepts_nested_attributes_for :topups, :allow_destroy => true
end

class PolicyItem < ActiveRecord::Base
  belongs_to :policy, :foreign_key => "policy_id"
end

Then i add debug statement on every callback to verify the sequence between the parent and nested object and the result is :-

• Policy before_validation callback
• Policy validate callback
• PolicyItem Topup before_validation callback
• PolicyItem Topup validate callback
• PolicyItem Topup after_validation callback
• PolicyItem BasicPlan before_validation callback
• PolicyItem BasicPlan validate callback
• PolicyItem BasicPlan after_validation callback
• Policy after_validation callback
• Policy before_save callback
• PolicyItem Topup before_save callback
• PolicyItem Topup after_save callback
• Policy after_save callback
• PolicyItem BasicPlan before_save callback
• PolicyItem BasicPlan after_save callback

As the result shown, the validation callback sequence is fine but the save callback sequence is a bit strange. The parent object's after_save callback has been invoked before the child's callback with has_one association. Child with has_many association seems behave normally as expected. Not sure it is by design or just a bug.