title: Prepare hash for "grouped_options_for_select"
body: <p>I'm trying to prepare a hash for a select on my view.</p>
  
  <p>For the moment I have this ugly thing.</p>
  
  <pre><code>@platforms = {}
  Platform.all.each do |platform| 
    @platforms[platform.name] = platform.game_updates
                                        .includes(:game)
                                        .where(game: {store_id: current_store.id})
                                        .map {|game_update| [game_update.name, game_update.game.id] }
  end
  </code></pre>
  
  <p>I want to have something like this :</p>
  
  <pre><code>@platforms = {
    'Plateform America' =&gt; [['Fallout4',76896], ['Just',56774]],
    'Plateform Ed' =&gt; [[Fallout3',77445],['Fallout2',75674],['Mario',45677]]
  }
  </code></pre>
  
  <p>I have two problems. <code>Game</code> can have many <code>GameUpdates</code>. And for the moment the list show all the game_updates. It's like having <code>Fallout4.1</code>, <code>Fallout4.2</code>, <code>Fallout4.3</code> and I only want the last <code>Games_update</code> of <code>Game</code>. </p>
  
  <p>Other problem is to refactor the first code. I've try to play with <code>group_by</code> without success. </p>
  
  <p>Thanks</p>