rosa-build/app/models/build_list_observer.rb

class BuildListObserver < ActiveRecord::Observer
  observe :build_list

  def before_update(record)
    if record.status_changed?
      record.started_at = Time.now if record.status == BuildServer::BUILD_STARTED
      if [BuildServer::BUILD_ERROR, BuildServer::SUCCESS].include? record.status
        # stores time interval beetwin build start and finish in seconds
        record.duration = record.current_duration
        
        if record.status == BuildServer::SUCCESS
          # Update project average build time
          av_time = record.project.average_build_time
          n = record.project.build_count
          new_av_time = ( av_time * n + record.duration ) / ( n + 1 )
          record.project.update_attribute(:average_build_time, new_av_time)
          record.project.update_attribute(:build_count, n + 1 )
        end  
      end
    end
  end
end