is_reviewable
... => 2
@post.average_rating # => 2.0, i.e. don't count nil (a.k.a. "no opinion")
@post.unreview!(:by ... UI...).
Example: @app/views/reviews/rate.js.rjs@
page.replace_html '#rating_wrapper', render('/reviews/rating', :reviewable => @post, :reviewer => ...