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