Mongodb - Find count of distinct items after applying aggregate and match - node.js

Question

Trying to figure out something from Mongo using mongoose in optimal way.
I have following documents
Regions
{
"_id" : ObjectId("5cf21263ff605c49cd6d8016"),
"name" : "Asia"
}
Countries can be part of multiple regions
{
"_id" : ObjectId("5d10a4ad80a93a1d7cd56cc6"),
"regions" : [
ObjectId("5d10a50080a93a1d7cd56cc7"),
ObjectId("5cf2126bff605c49cd6d8017")
],
"name" : "India"
}
Places belongs to one country
{
"_id" : ObjectId("5d11bb8180a93a1d7cd56d26"),
"name" : "Delhi",
"country" : ObjectId("5d136e7a4e480863a51c4056"),
}
Programs each in dayshows array represents one day. On a day show can cover multiple places.
{
"_id" : ObjectId("5d11cc9480a93a1d7cd56d31"),
"dayshows" : [
{
"_id" : ObjectId("5d11cc9480a93a1d7cd56d41"),
"places" : [
ObjectId("5d11bb8180a93a1d7cd56d26")
],
},
{
"_id" : ObjectId("5d11cc9480a93a1d7cd56d3c"),
"places" : [
ObjectId("5d11bb8180a93a1d7cd56d26"),
ObjectId("5d11bc7c80a93a1d7cd56d2e")
]
}
]
}
What am I trying to figure out?
For a given region, for each country in region which all places are covered and count of programs for each place. Using nodejs and mongoose.
Example
Input - Asia
Output
India
- Delhi (3)
- Mumbai (5)
Thailand
- Pattaya (2)
- Bangkok (5)
New to mongo.

Answer 1

You need to use $lookup to cross different collections.
Pipeline:
Stages 1-6 serves to get all related data.
(Optional) Stages 7-10 serves to transform aggregated data into key:pair object.
ASSUMPTION
Programs to visit 2 places counted as is (Place1: +1, Place2: +1)
You know how to execute MongoDB aggregation in node.js
db.Regions.aggregate([
{
$match: {
name: "Asia"
}
},
{
$lookup: {
from: "Countries",
let: {
region: "$_id"
},
pipeline: [
{
$match: {
$expr: {
$in: [
"$$region",
"$regions"
]
}
}
},
{
$lookup: {
from: "Places",
localField: "_id",
foreignField: "country",
as: "Places"
}
}
],
as: "Countries"
}
},
{
$unwind: "$Countries"
},
{
$unwind: "$Countries.Places"
},
{
$lookup: {
from: "Programs",
localField: "Countries.Places._id",
foreignField: "dayshows.places",
as: "Countries.Places.Programs"
}
},
{
$project: {
"name": 1,
"Countries.name": 1,
"Countries.Places.name": 1,
"Countries.Places.Programs": {
$size: "$Countries.Places.Programs"
}
}
},
{
$group: {
_id: {
name: "$name",
Countries: "$Countries.name"
},
Places: {
$push: {
k: "$Countries.Places.name",
v: "$Countries.Places.Programs"
}
}
}
},
{
$project: {
_id: 1,
Places: {
$arrayToObject: "$Places"
}
}
},
{
$group: {
_id: "$_id.name",
Countries: {
$push: {
k: "$_id.Countries",
v: "$Places"
}
}
}
},
{
$project: {
_id: 0,
name: "$_id",
Countries: {
$arrayToObject: "$Countries"
}
}
}
])
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