Find the average value in MongoDB from JSON - json

Question

In my MongoDB (export from JSON file) I have database "dab" with structure like this:
id:"1"
datetime:"2020-05-08 5:09:56"
name:"namea"
lat:55.826738
lon:45.0423412
analysis:"[{"0":0.36965591924860347},{"5":0.10391287134268598},{"10":0.086884394..."
I'm using that db for spark analysis via MongoDB-Spark Connector.
My problem is field "analysis" - I need average result for all values from every interval ("0", "5", "10", ..., "1000"), so I have to sum 0.36965591924860347 + 0.10391287134268598 + 0.086884394 + ... and divide by number of intervals (I have 200 intervals in every column), and finally multiply the result by 100.

Answer 1

My solution would be this one:
db.collection.aggregate([
{
$set: {
analysis: {
$map: {
input: "$analysis",
in: { $objectToArray: "$$this" }
}
}
}
},
{
$set: {
analysis: {
$map: {
input: "$analysis",
in: { $first: "$$this.v" }
}
}
}
},
{ $set: { average: { $multiply: [ { $avg: "$analysis" }, 100 ] } } }
])
Mongo playground

Answer 2

You can use $reduce on that array,sum the values,and then divide with the number of elements and then multiply with 100
db.collection.aggregate([
{
"$addFields": {
"average": {
"$multiply": [
{
"$divide": [
{
"$reduce": {
"input": "$analysis",
"initialValue": 0,
"in": {
"$let": {
"vars": {
"sum": "$$value",
"data": "$$this"
},
"in": {
"$add": [
"$$sum",
{
"$arrayElemAt": [
{
"$arrayElemAt": [
{
"$map": {
"input": {
"$objectToArray": "$$data"
},
"as": "m",
"in": [
"$$m.k",
"$$m.v"
]
}
},
0
]
},
1
]
}
]
}
}
}
}
},
{
"$size": "$analysis"
}
]
},
100
]
}
}
}
])
You can test the code here
But this code has 1 problem, you save data in documents, and MongoDB
doesn't have a function like get(document,$$k), the new MongoDB v5.0 has a $getField but still accepts only constants no variables.
I mean we cant do in your case getField(doc,"5").
So we have the cost of converting each document to an array.