073_invalidate_cached_unread_posts_counts_and_generate_all_to_speed_up_load.rb

class InvalidateCachedUnreadPostsCountsAndGenerateAllToSpeedUpLoad < ActiveRecord::Migration
  
  def self.up
    DiscussionGroupUnreadPostsCount.delete_all
    
    all_users = User.find(:all)
    
    all_users.in_groups_of(100) do |users|
      fork do
        # reconnect because forking rapes the connection pool
        ActiveRecord::Base.connection.reconnect!
        
        users.each do |u|
          if u # in_groups_of gives nils instead of truncating the array
            start_time = Time.now
            discussion_groups = u.discussion_groups # not every user is part of every discussion group
            
            discussion_groups.each do |dg|
              # creates a count cache for this DiscussionGroup/User pair
              DiscussionGroupUnreadPostsCount.create(:discussion_group_id => dg.id, :user_id => u.id)
              # after_create handles counting the number of unread posts for the pair
            end
            
            puts "Created caches for User:#{u.id.to_s} for #{discussion_groups.size.to_s} [%.5fms]" % ((Time.now - start_time) * 1000.0)
          end
        end
      end
    end
    
    Process.waitall # wait for all processes to complete, could add error checking
    # otherwise the migration will finish before the subprocesses
    
    # odds are that our connection went away while waiting, so reconnect
    ActiveRecord::Base.connection.reconnect!
    # otherwise the schema won't be able to be updated when all the processes
    # ends since it disconnected
  end
  
  def self.down
    # nothing
  end
  
end