How do I optimize the queries for this?

post.rb

class Post < ActiveRecord::Base
  before_create :update_counters!
  
  def update_counters!
    now = Time.zone.now
    
    %w(draft published).each do |status|
      [nil, "site_scoped", "category_scoped"].each do |scoping|
        # "", "site_scoped_", "category_scoped_"
        scoping = scoping.nil? ? "" : "#{scoping}_"
        
        # last_published_post, etc.
        post    = send("#{scoping}last_#{status}_post")
        
        if post.present? && now >= post.created_at
          # seconds_since_last_published_post
          self.send("#{scoping}seconds_since_last_#{status}_post=", (now - post.created_at).to_i)
        end
      end
    end
  end
  
  %w(draft published).each do |status|
    class_eval %{
      def last_#{status}_post
        @last_#{status}_post ||= ::Post.first(:order => "created_at desc", :conditions => {:user_id => user_id, :status => "#{status}"})
      end
      
      def category_scoped_last_#{status}_post
        @category_scoped_last_#{status}_post ||= ::Post.first(:order => "created_at desc", :conditions => {:user_id => user_id, :status => "#{status}", :category_id => category_id})
      end
      
      def site_scoped_last_#{status}_post
        @site_scoped_last_#{status}_post ||= ::Post.first(:order => "created_at desc", :conditions => {:user_id => user_id, :status => "#{status}", :site_id => site_id})
      end
    }
  end
  
  def last_post
    unless @last_post.present?
      if last_draft_post && last_published_post
        @last_post = last_draft_post.id > last_published_post.id ? last_draft_post : last_published_post
      else
        @last_post = last_draft_post || last_published_post
      end
    end
    
    @last_post
  end
  
  def site_scoped_last_post
    unless @site_scoped_last_post.present?
      if site_scoped_last_draft_post && site_scoped_last_published_post
        @site_scoped_last_post = site_scoped_last_draft_post.id > site_scoped_last_published_post.id ? site_scoped_last_draft_post : site_scoped_last_published_post
      else
        @site_scoped_last_post = site_scoped_last_draft_post || site_scoped_last_published_post
      end
    end
    
    @site_scoped_last_post
  end
  
  def category_scoped_last_post
    unless @category_scoped_last_post.present?
      if category_scoped_last_draft_post && category_scoped_last_published_post
        @category_scoped_last_post = category_scoped_last_draft_post.id > category_scoped_last_published_post.id ? category_scoped_last_draft_post : category_scoped_last_published_post
      else
        @category_scoped_last_post = category_scoped_last_draft_post || category_scoped_last_published_post
      end
    end
    
    @category_scoped_last_post
  end
end