Mongodb project results into single document - mongodb

Question

I have a collection like this:
{
"_id" : ObjectId("5f4e81f1da5ea3cb7c248a8f"),
"type" : "TYPE_1",
"updateTime" : ISODate("2020-08-24T11:10:43.219+0000")
}
{
"_id" : ObjectId("5f4e8206da5ea3cb7c248a90"),
"type" : "TYPE_1",
"updateTime" : ISODate("2020-09-24T11:10:43.219+0000")
}
{
"_id" : ObjectId("5f4e821fda5ea3cb7c248a91"),
"type" : "TYPE_2",
"updateTime" : ISODate("2020-09-25T11:10:43.219+0000")
}
I want to know how many documents there are of each type and also obtain the date of the last global modification. For now I can get these results like this:
db.getCollection("test").aggregate(
// Pipeline
[
// Stage 1
{
$group: {
_id : "$type",
count: { $sum: 1 },
lastUpdate: { "$max": "$updateTime" }
}
},
// Stage 2
{
$sort: {
lastUpdate : -1
}
},
]
);
With which I get the results this way:
{
"_id" : "TYPE_2",
"count" : 1.0,
"lastUpdate" : ISODate("2020-09-25T11:10:43.219+0000")
}
{
"_id" : "TYPE_1",
"count" : 2.0,
"lastUpdate" : ISODate("2020-09-24T11:10:43.219+0000")
}
So I have both the sum of each document and the last modification (thanks to the sort).
But I would like to project and get the results like this, in a single result document:
{
"type1" : 2.0,
"type2" : 1.0,
"lastUpdate" : ISODate("2020-09-25T11:10:43.219+0000")
}

Answer 1

#varman's answer is good, this is just in different way,
$group you have already done by your self
$group create types array to combine all documents
$replaceWith to replace root with field types to convert $arrayToObject
db.collection.aggregate([
{
$group: {
_id: "$type",
count: { $sum: 1 },
lastUpdate: { $max: "$updateTime" }
}
},
{
$group: {
_id: null,
types: {
$push: {
k: "$_id",
v: "$count"
}
},
lastUpdate: { $max: "$lastUpdate" }
}
},
{
$replaceWith: {
$mergeObjects: [
{ lastUpdate: "$lastUpdate" },
{ $arrayToObject: "$types" }
]
}
}
])
Playground

Answer 2

You can use following stages after your stage.
{
$group: {
_id: null,
data: {
$push: {
type: "$_id",
count: "$count"
}
},
lastUpdate: {
$first: "$lastUpdate"
}
}
},
{
$project: {
data: {
$arrayToObject: {
$map: {
input: "$data",
in: {
k: "$$this.type",
v: "$$this.count"
}
}
}
},
lastUpdate: 1
}
},
{
$addFields: {
"data.lastUpdate": "$lastUpdate"
}
},
{
"$replaceRoot": {
"newRoot": "$data"
}
}
Working Mongo playground