Rails 4 redirect_to by condition

Question

I have an action create in Comments controller like this

def create
    @comment = Comment.new(comment_params)
    @comment.user = current_user
    @comment.post = @post
    if @comment.save
      flash[:notice] = 'Comment was successfully created!'
      redirect_to root_path
    else
      flash[:alert] = "Cant add comment.\n#{@comment.errors.full_messages.join(',')}"
      redirect_to root_path
    end
  end

The request to this action goes from form on index page. Now I want to use the same form on show page. Question: how make condition like this:

 if @comment.save
   if something
    redirect_to show_path
   else
    redirect_to root_path
   end
 else
  if something
    redirect_to show_path
  else
    redirect_to root_path
  end
 end