Ruby on Rails

I have the following models

    class Parent < ActiveRecord::Base

      has_many :children

    end

    class Child < ActiveRecord::Base

     belongs_to :parent

    end

What I want to do is changing a child object by parent.children.find_or_initialize_by() and modify the child object, then save it by parent.save. But the children doens't get saved. Here's the code.

    # prepare a parent with a child

    p = Parent.new({:name=>'parent'})

    p.children << Child.new({:name => 'child'})

    p.save

    # modify and save child

    p = Parent.first

    c = p.children.find_or_initialize_by({:name=>'child'}) #=> returns the child already created above

    c.name = 'new child'

    p.save #=> children aren't saved

I know this is because find_by method always returns a new object rather than the one already loaded. But it seems more natural to me if the code above works because the method is invoked through ActiveRecord::Associations::CollectionProxy. Am I missing anything to get this to work? Or are there any reasons that find_by on ActiveRecord::Associations::CollectionProxy have to work as it currently does?

(I know I can save the modified child by c.save, but I want to save the child through parant.save, because I want the errors on child object to be referenced by parent.errors.)