# Two ways to do this...pretty much equivalent on effort, so pick the one that reads
# more smoothly to you
 
# Alternative #1 
 def create
    @topic = Topic.new(params[:topic])
# next line was your original problem, I think - you're not passing an id param, you're
# passing a game_id nested inside of topic
    @topic.game_id = params[:topic][:game_id]
    if @topic.save
      flash[:notice] = "Successfully created topic."
      redirect_to ([@game, @topic])
    else
      render :action => 'new'
    end
  end
 
# Alternative #2
  def create
    @game = Game.find(params[:topic][:game_id])
# switched to using build here - create saves, build doesn't
    @topic = @game.topics.build(params[:topic])
    if @topic.save
      flash[:notice] = "Successfully created topic."
      redirect_to ([@game, @topic])
    else
      render :action => 'new'
    end
  end