named_scope :between, lambda {|starting, ending|
    conditions = ['']
    conditions[0] += '(started_at > ? AND started_at < ?)'
    conditions << starting
    conditions << ending
    conditions[0] += ' OR (completed_at IS NULL AND stopped_at IS NULL AND end_at > ? AND end_at < ?)'
    conditions << starting
    conditions << ending
    conditions[0] += ' OR (completed_at IS NOT NULL AND completed_at > ? AND completed_at < ?)'
    conditions << starting
    conditions << ending
    conditions[0] += ' OR (stopped_at IS NOT NULL AND stopped_at > ? AND stopped_at < ?)'
    conditions << starting
    conditions << ending
    {:conditions => conditions}
  }