title: increment value in a hash
body: <p>I have a bunch of posts which have category tags in them. 
  I am trying to find out how many times each category has been used. </p>
  
  <p>I'm using rails with mongodb, BUT I don't think I need to be getting the occurrence of categories from the db, so the mongo part shouldn't matter. </p>
  
  <p>This is what I have so far</p>
  
  <pre>
  @recent_posts = current_user.recent_posts #returns the 10 most recent posts
  @categories_hash = {'tech' => 0, 'world' => 0, 'entertainment' => 0, 'sports' => 0}
      @recent_posts do |cat|
         cat.categories.each do |addCat|
           @categories_hash.increment(addCat) #obviously this is where I'm having problems
         end
       end
  end
  </pre>
  
  <p>the structure of the post is</p>
  
  <pre>{"_id" : ObjectId("idnumber"), "created_at" : "Tue Aug 03...", "categories" :["world", "sports"], "message" : "the text of the post", "poster_id" : ObjectId("idOfUserPoster"), "voters" : []}</pre>
  
  <p>I'm open to suggestions on how else to get the count of categories, but I will want to get the count of voters eventually, so it seems to me the best way is to increment the categories_hash, and then add the voters.length, but one thing at a time, i'm just trying to figure out how to increment values in the hash. </p>