_ como es ahora (mas o menos):

%ul
  - for activity in @user.activity_feed
    %li= render :partial => "activity", :object => activity

activities_helper.rb

def render_activity(activity)
  partial = case activity.activity_type
    when :comment then "activities/comment"
    else               "activities/generic"
  end
  render :partial => partial, :object => activity
end

activity_feed.haml

%ul
  - for activity in @user.activity_feed
    %li= render_activity(activity)