Topic: Add field to acts_as_taggable?

I'm trying to implement acts_as_taggabe so that the tags model has an additional field. You can see this is reflected in the schema.

  create_table "tags", :force => true do |t|
    t.column "project_id", :integer
    t.column "name",       :string

I see that the recommended method looks something like this and adds the 'tags' into the string.
@rateme.tag_with params[:tags] if params[:tags]

Here's an example of my trying to put this value in, but I always get the message that it doesn't recognize the 'project_id' method?
    @rateme.tag_with params[:tags] if params[:tags]
    @rateme.tag.project_id = @portal.project_id

As Tags are parents of Taggings and Tagging are linked to the operative record 'rateme', should I be accessing the Tag values through Taggings?
I know this must sound like a 'jumble' from an unclear mind but I hope you see the 'flavor' of this question through the 'facts'.