Unable to Clear a :through Relationship w/ clear or replace

I have the following setup:

class NewsItem < ActiveRecord::Base
   has_many :connections, :as => :connectable, :dependent => :destroy
   has_many :ideas, :through => :connections
end

The relationship is a has_many :through that is also polymorphic.

I can add ideas to news items with this code:

@news_item.ideas.push(related_ideas)

related_ideas is an array of ideas.

The problem comes when I try to update the ideas for an existing news item.

First I tried this:

@news_item.ideas.replace(new_ideas)

This literally does nothing - no log output at all, no DB activity.

Then I tried this:

@news_item.ideas.clear
@news_item.ideas.push(new_ideas)

Again, nothing at all. No error, nada. I would have expected the 'clear' method call to remove the records that associate the news items and the ideas.

What am I doing wrong?

Thanks,
Hunter