Calculate price with mongo aggregate query - Evaluate array of dates with array of prices

gistfile1.txt

// Simple document example
// Every boat contains array of prices for different days
// We decided to use simple concatenated date int to evaluate date ranges, f.e. date '2017-08-24' becomes integer 20170824
// If user wants to reservate boat in dates when prices is not defined, it will not be shown in results
Boat = {
  _id : ObjectId("100000000000000000000001"),
  prices : [
    // price available for august
    {
      price : NumberInt(100),
      check_in : NumberInt(20170801),
      check_out : NumberInt(20170831)
    },
    // price available for september
    {
      price : NumberInt(50),
      check_in : NumberInt(20170901),
      check_out : NumberInt(20170930)
    }
  ]
}

// period: 
// -- we used to generate price for every day differently
// -- dates array is generated automatically from user's checkIn and checkOut dates
// -- dates array contains minimum 1 day, and maximum 28 days
// -- last date of reservation is not calculated - user returns boat and it's available for another user
// example: user wants to rent a yacht for 1 week in august, from 12. to 19.

var period = [20170812,20170813,20170814,20170815,20170816,20170817,20170818];

db.boats.aggregate([
  {
    $project : {
      data : {
        _id : '$_id',
        dates : period,
        prices : {
          $map : {
            input : period,
            as : 'date',
            in : {
              // $filter returns array, we need to get data from it's index 0
              $arrayElemAt : [{
                $filter : {
                  input : '$prices',
                  as : 'price',
                  cond : {
                    $and : [
                      { $gte : ['$$date', '$$price.check_in']},
                      { $lte : ['$$date', '$$price.check_out']}
                    ]
                  }
                }
              }, 0]
            }
          }
        }
      } 
    }
  },
  {
    $match : {
      // if boat doesn't have prices set for user's checkIn & checkOut dates, $filter returns null
      'data.prices' : { $ne : null }
    }
  },
  {
    // group the results
    $group : {
			_id : '$data._id',
			prices : { $first : '$data.prices' }
    }
  },
  {
    // $prices contains array of objects, we would like to return only price integers
    $unwind : '$prices'
  },
  {
    // now $prices is an array of integers (prices) per day
    // totalPrice is sum of all $prices
		$group : {
			_id : '$_id',
			prices : { $push : '$prices.price' },
			totalPrice : { $sum : '$prices.price' }	
		}
	}
]);