Do count with condition inside mongDB aggregation's function - mongodb

Question

I have 2 collections inside my MongoDB:
Order:
{
"date" : ISODate("2020-07-30T22:00:00.000Z"),
"item" : "banana",
"price": "4$",
"order_type" : "INTERNATIONAL", // There are 2 order types: INTERNATIONAL and MAINLAND
"user" : { // I use a dbref to the User collection
"$ref" : "user",
"$id" : "user_0"
}
}
User:
{
"_id": "user_0"
"login" : "user1",
"password" : "$2a$10$mE.qmcV0mFU5NcKh73TZx.z4ueI/.bDWbj0T1BYyqP481kGGarKLG",
"first_name" : "Henry",
"last_name" : "Windsor",
"email" : "hw#gmail.com",
}
Each order contains a DB reference to the corresponding user who made it. This is my mongo code to calculate the total number of orders that each user makes.
db.getCollection('order').aggregate([
{$group: {
_id: '$user',
totalNbOfOrders: {$sum: 1}
}},
{$addFields: {foreign_key: {$objectToArray: "$_id"}}},
{$lookup: {
from: 'user',
localField: 'foreign_key.1.v',
foreignField: '_id',
as: 'userInfo'
}
},
{ $unwind: '$userInfo'},
{ $project: {
'_id': 0,
'first_name': '$userInfo.first_name',
'last_name': '$userInfo.last_name',
'totalNbOfOrders': '$totalNbOfOrders'
}
}
])
And the result is:
/* 1 */
{
"first_name" : "John",
"last_name" : "Kennedy",
"totalNbOfOrders" : 2.0
}
/* 2 */
{
"first_name" : "Peter",
"last_name" : "Parker",
"totalNbOfOrders" : 4.0
}
/* 3 */
{
"first_name" : "Bruce",
"last_name" : "Banner",
"totalNbOfOrders" : 2.0
}
Now, what I also want to calculate is the number of international orders (and eventually of mainland orders) that each user made to have something like this:
{
"first_name" : "Tony",
"last_name" : "Stark",
"totalNbOfOrders" : 10.0,
"totalNbOfInternationalOrders": 4.0
"totalNbOfMainlandOrders": 6.0
}
I haven't figured out how to write the code.
I tried to use "$accumulator" operator (new feature in version 4.4 of MongoDB) inside "$group" but I used MongoDB 4.2.7, I have to use operators from older versions to accomplish this. Does anybody know how to solve this problem?

Answer 1

You can do it inside $group, using $cond and $eq,
{
$group: {
... // skipped
// INTERNATIONAL COUNT
totalNbOfInternationalOrders: {
$sum: {
$cond: {
if: {
$eq: ["$order_type", "INTERNATIONAL"]
},
then: 1,
else: 0
}
}
},
// MAINLAND COUNT
totalNbOfMainlandOrders: {
$sum: {
$cond: {
if: {
$eq: ["$order_type", "MAINLAND"]
},
then: 1,
else: 0
}
}
}
}
},
set in $project
{
$project: {
... // skipped
"totalNbOfInternationalOrders": "$totalNbOfInternationalOrders",
"totalNbOfMainlandOrders": "$totalNbOfMainlandOrders"
}
}
Playground: https://mongoplayground.net/p/_IeVcSFt_nY