Aggregate group by array and divide quantity to array length - arrays

Question

Now I want to aggregate schema to group by users in array and divide items field to array length to create average..
This is simple json data ->
[{"users": ["5ea40086fc4b145b489da93d","5e8cb9a4462e45178c4d3405"],"isBuilt": true, "_id": "5eadd43b30f97f342cf663fc", "items": 3, ...},
{"users": ["5e8cb9a4462e45178c4d3405"], "isBuilt": true, "_id": "5ead419081eec52258b67f70", "items": 5, ...}]
And after aggregating with ->
Building.aggregate([
{
$match: {
updatedAt: {
$gte: startDate,
$lte: endDate
},
isBuilt: true
}
},
{
$unwind: "$users"
},
{
$group: {
_id: "$users",
items: {
$sum: '$items'
}
}
},
{
$project: {
user: '$_id',
items: 1,
_id: 0
}
}
])
I got this json ->
[{"items": 3, "user": "5ea40086fc4b145b489da93d"}, {"items": 8, "user": "5e8cb9a4462e45178c4d3405"}]
As you see here I got sum of items. In initial data Users "5ea40086fc4b145b489da93d" and "5e8cb9a4462e45178c4d3405" have 3 items, and user "5e8cb9a4462e45178c4d3405" has 5 items. And after aggregating they count by sum of items, that user "5e8cb9a4462e45178c4d3405" -> 8 items, and user "5ea40086fc4b145b489da93d" -> 3 items... Now I want make average items to users, like if length of array users is 2 or more it will divide items and give sum.. and final json will look like ->
[{"items": 1.5, "user": "5ea40086fc4b145b489da93d"}, {"items": 6.5, "user": "5e8cb9a4462e45178c4d3405"}]
PS if result of item is not integer, result should be rounded to ten

Answer 1

I've solved my problem with aggregation ->
Building.aggregate([
{
$match: {
updatedAt: {
$gte: startDate,
$lte: endDate
},
isBuilt: true
}
},
{
$addFields: {
itemsAvg: {
$divide: ["$items", {$size: "$users"}]
}
}
},
{
$addFields: {
roundedItemsAvg: {
$round: ["$itemsAvg", 1]
}
}
},
{
$unwind: "$users"
},
{
$group: {
_id: "$users",
items: {
$sum: '$roundedItemsAvg'
}
}
},
{
$project: {
user: '$_id',
items: 1,
_id: 0
}
}
])