Get average per year and label? - mongodb

Question

I can use this query to get the average sqmPrice for a myArea
db.getCollection('sold').aggregate([
{$match:{}},
{$group: {_id: "$myArea", "sqmPrice": {$avg: "$sqmPrice"} }}
])
Output:
[
{
"_id" : "Yttre Aspudden",
"sqmPrice" : 48845.7777777778
},
{
"_id" : "Hägerstensåsen",
"sqmPrice" : 120
}
]
I would like to group this by year, ideally an object that looks like this:
{
"Yttre Aspudden": {
2008: 1232,
2009: 1244
...
}
...
}
but the formatting is not the most important.
Here is a sample object, I would like to use soldDate:
{
"_id" : ObjectId("5beca41c78f21248ab47f4a6"),
"location" : {
"address" : {
"streetAddress" : "Ljusstöparbacken 26C"
},
"position" : {
"latitude" : 59.31427884,
"longitude" : 18.00892421
},
"namedAreas" : [
"Hägersten-Liljeholmen"
],
"region" : {
"municipalityName" : "Stockholm",
"countyName" : "Stockholms län"
},
"distance" : {
"ocean" : 3777
}
},
"listPrice" : 1895000,
"rent" : 1959,
"floor" : 1,
"livingArea" : 38.5,
"source" : {
"name" : "Fastighetsbyrån",
"id" : 1573,
"type" : "Broker",
"url" : "http://www.fastighetsbyran.se/"
},
"rooms" : 1.5,
"published" : ISODate("2018-11-02T20:55:19.000Z"),
"constructionYear" : 1959,
"objectType" : "Lägenhet",
"booliId" : 3278478,
"soldDate" : ISODate("2018-11-14T00:00:00.000Z"),
"soldPrice" : 2620000,
"soldPriceSource" : "bid",
"url" : "https://www.booli.se/annons/3278478",
"publishedDays" : 1735,
"soldDays" : 1747,
"daysUp" : 160,
"street" : "Ljusstöparbacken",
"streetYear" : "Ljusstöparbacken Hägersten-Liljeholmen 1959",
"yearDay" : 318,
"yearWeek" : 46,
"roughSize" : 40,
"sqmPrice" : 49221,
"myArea" : "Gröndal",
"hotlist" : true
}

Answer 1

You need to generate your keys dynamically so you have to use $arrayToObject. To build an object which aggregates the data you need three $group stages and to create new root of your document you can use $replaceRoot, try:
db.sold.aggregate([
{ $group: {_id: { area: "$myArea", year: { $year: "$soldDate" } }, "sqmPrice": {$avg: "$sqmPrice"} }},
{ $group: { _id: "$_id.area", avgs: { $push: { k: { $toString: "$_id.year" }, v: "$sqmPrice" } } } },
{ $group: { _id: null, areas: { $push: { k: "$_id", v: { $arrayToObject: "$avgs" } } } } },
{ $replaceRoot: { newRoot: { $arrayToObject: "$areas" } } }
])