MongoDb: Match value on foreign collection - mongodb

Question

I have the collection USER:
{
"_id" : ObjectId("5d64d2bf48dd17387d77d27a"),
"name" : "John",
"notifications" : {
"email" : false,
"sms" : true
}
},
{
"_id" : ObjectId("5da9586911e192081ee1c6be"),
"name" : "Mary",
"notifications" : {
"email" : false,
"sms" : false
}
}
And this other collection ALERT:
{
"_id" : ObjectId("5d54f04dbe5e6275e53a551e"),
"active" : true,
"user_id" : ObjectId("5d64d2bf48dd17387d77d27a")
},
{
"_id" : ObjectId("5d54f04dbe5e6275e53a551f"),
"active" : false,
"user_id" : ObjectId("5d64d2bf48dd17387d77d27a")
},
{
"_id" : ObjectId("5d54f04dbe5e6275e53a552e"),
"active" : true,
"user_id" : ObjectId("5da9586911e192081ee1c6be")
},
{
"_id" : ObjectId("5d54f04dbe5e6275e53a552f"),
"active" : true,
"user_id" : ObjectId("5da9586911e192081ee1c6be")
}
I want a MongoDB query that lists the documents on collection ALERT that have property "active" as TRUE and whose matching USER has element "sms" on property "notifications" as TRUE too.

Answer 1

You can use Uncorelated sub queries in $lookup
$match to get the "notifications.sms": true
$lookupto join two collections. We are assigning uId = _id from USER collection. Inside the pipeline, we use $match to find the active :true, and _id=uId
here is the script
db.USER.aggregate([
{
"$match": {
"notifications.sms": true
}
},
{
"$lookup": {
"from": "ALERT",
"let": {
uId: "$_id"
},
"pipeline": [
{
$match: {
$and: [
{
active: true
},
{
$expr: {
$eq: [
"$user_id",
"$$uId"
]
}
}
]
}
}
],
"as": "joinAlert"
}
}
])
Working Mongo playground