How do you use anchors for IDs in routes in Rails 3?

You could simply use

redirect_to post_path(comment.post, :anchor => "comment-#{comment.id}")

to manually build the URL with the anchor. That way, you can still have the absolute URL to your comments as posts/:post_id/comments/:comment_id in your routes. You can also create a helper method in e.g. application_controller.rb

class ApplicationController
  helper :comment_link

  def comment_link(comment)
    post_path(comment.post, :anchor => "comment-#{comment.id}")
  end
end