Mongo DB query for combining sorting and grouping - database

Question

Let's say I have a collection with the following (dummy) data:
Country
State
Population Density (people per km^2)
Cases (in millions)
USA
New York
161
1.03
USA
California
95
4.47
Germany
Berlin
4,227
0.19
India
Kerala
859
4.09
India
Karnataka
319
2.95
India
Maharashtra
370
6.47
What is an optimized mongodb query to get results grouped by country and sorted by the count of states, also each group should contain states sorted by 'Cases'?
The results should look like this in JSON
{
'results': [
{
'country' : 'India',
'num_states': 3,
'states': [
{
'State': 'Maharashtra',
'Cases': 6.47,
'PPD' : 370,
},
{
'State': 'Kerala',
'Cases': 4.09,
'PPD' : 859,
},
{
'State': 'Karnataka',
'Cases': 2.95,
'PPD' : 319,
}
]
},
{
'country' : 'USA',
'num_states': 2,
'states': [
{
'State': 'California',
'Cases': 4.47,
'PPD' : 95,
},
{
'State': 'New York',
'Cases': 1.03,
'PPD' : 161,
}
]
},
{
'country' : 'Germany',
'num_states': 1,
'states': [
{
'State': 'Berlin',
'Cases': 0.19,
'PPD' : 4227,
}
]
},
]
}
Note: My actual data is different, but the use case is the same.

Answer 1

first sort by cases -1
then group by country
add new field for sorting
and project
db.collection.aggregate(
[
{
'$sort': {
'Cases': -1
}
}, {
'$group': {
'_id': {
'country': '$country'
},
'num_states': {
'$sum': 1
},
'states': {
'$push': {
'states': '$$ROOT.state',
'cases': '$$ROOT.Cases',
'ppd': '$$ROOT.Density'
}
}
}
}, {
'$addFields': {
'sorf': {
'$max': '$states.cases'
}
}
}, {
'$sort': {
'sorf': -1
}
}, {
'$project': {
'country': '$_id.country',
'num_states': '$num_states',
'states': 1,
'_id': 0
}
}
])

Answer 2

$sort - sort by cases in descending order
$group - group by country and construct an array of states, and get total count of states
$sort - sort by number of states in descending order
$project - rename the required fields
db.collection.aggregate([
{
"$sort": {
"cases": -1
}
},
{
$group: {
_id: "$country",
num_states: {
$sum: 1
},
states: {
$push: {
state: "$state",
cases: "$cases",
PPD: "$population"
}
}
}
},
{
"$sort": {
"num_states": -1
}
},
{
$project: {
country: "$_id",
num_states: 1,
states: 1,
_id: 0
}
}
])
Mongo Playgroud