Can't update one model from another model (polymorphic models)

Hi. I'm having a little trouble updating a model of mine. So I have a
Dimension model which is polymorphic. The 2 derived classes are Fact
and Opinion. In my Dimension model I have an after_save callback
function that creates an Opinion record whenever a Fact record is
created. So it looks like this:

In my fact controller, in the update action, I set the old_name to
whatever the fact's name was before its creation. So if the

def create_opinion_if_fact
  if valuable_type == 'Fact'
    op = Opinion.find_by_name @old_name

    unless op
      op = Opinion.new
      op.dimension = Dimension.new :name => name
    end

    op.dimension.name = name
    op.dimension.desc = desc

    # Give the opinions the same associations as the fact
    op.dimension.concepts.clear

    concepts.each do |c|
      op.dimension.concepts << c
    end

    op.save!
  end
end

In my facts_controller's update and create actions, I set
Dimension.old_name to Fact.find(params[:id]).dimension.name. Since
this will have the old name in it. Of course when creating a new fact,
this will be empty, so in that case (the create action) I set old_name
to params[:fact][:dimension_attributes][:name].

So when I create a new fact, the opinion gets created as well. When I
try and update a fact, I get a validation error saying that the 'name'
is already taken. But I don't understand why this happens because if I
update a fact and don't change a name, then the first line in my
create_opinion_if_fact function will find a fact that's already there,
and then the save! should just update it.

Also, I have "validates_uniqueness_of :name, :scope
=> :valuable_type, :case_sensitive => false" in my Dimension model.
Valuable is the polymorphic link in this case.

One other thing. When I get rid of op.save! at the end of that
function and replace it with op.save false, I get 2 opinions created
for every fact. When I change it to just op.save, then everything
works fine (because it doesn't throw an exception and fails silently
instead).

Thanks for any help