# comments_controller

  def create
    @commentable = find_commentable
    @comment = @commentable.comments.build(params[:comment])

    if @comment.save
      redirect_to @comment.commentable
    else
      render :action => "show", :id => @commentable.id # <- how can i render the action for the right model, say post?
    end
  end

# comments/form

<% form_for [@commentable, Comment.new] do |f| %>
	<%= f.error_messages %>

# error

Template is missing

Missing template comments/show.erb in view path /app/views: