MongoDB aggregation: average sales per hour - mongodb

Question

I have a collection with sales. Now I need to get the average number of sales per hour within a date range.
Up to now I have a query like this:
db.getCollection('sales').aggregate({
"$match": {
$and: [
{ "createdAt": { $gte: ISODate("2018-05-01T00:00:00.000Z") } },
{ "createdAt": { $lt: ISODate("2018-10-30T23:59:00.000Z") } },
]
}
},{
"$project": {
"h":{"$hour":"$createdAt"},
}
},{
"$group":{
"_id": "$h",
"salesPerHour": { $sum: 1 },
},
},{
"$sort": { "salesPerHour": -1 }
});
The result looks like this: {"_id" : 15, "salesPerHour" : 681.0}
How can I get the average value of salesPerHour instead the sum?
Update 1 => Example document.
{
"_id" : "pX6jj7j4274J9xpSA",
"idFiscalSale" : "48",
"documentYear" : "2018",
"paymentType" : "cash",
"cashReceived" : 54,
"items" : [...],
"customer" : null,
"subTotal" : 23.89,
"taxTotal" : 3.7139,
"total" : 23.89,
"rewardPointsValue" : 0,
"rewardPointsEarned" : 24,
"discountValue" : 0,
"createdAt" : ISODate("2018-04-24T00:00:00.201Z")
}

Answer 1

You can use below aggregation query.
db.sales.aggregate([
{"$match":{
"createdAt":{
"$gte":ISODate("2018-05-01T00:00:00.000Z"),
"$lt":ISODate("2018-10-30T23:59:00.000Z")
}
}},
{"$group":{
"_id":{"$hour":"$createdAt"},
"salesPerHour":{"$sum":1}
}},
{"$group":{
"_id":null,
"salesPerHour":{"$avg":"$salesPerHour"}
}}
])

Answer 2

You can try below aggregation
You have to use $avg aggregation operator with the salesPerHour field
db.collection.aggregate([
{ "$match": {
"$and": [
{ "createdAt": { "$gte": ISODate("2018-05-01T00:00:00.000Z") }},
{ "createdAt": { "$lt": ISODate("2018-10-30T23:59:00.000Z") }}
]
}},
{ "$group": {
"_id": { "$hour": "$createdAt" },
"salesPerHour": {
"$avg": "$salesPerHour"
}
}}
])