.group({
        _id: 0,
        returnedBooks: { $sum: 1 },
        totalTime: { $sum: '$loanTimeHours' },
        avgLoanTimeHours: { $avg: '$loanTimeHours' },
        avgLoanTimeDays: { $avg: {
              $divide: [
                  '$loanTimeHours',
                  24
              ]}
        }
      })
      .exec();