How to compare fields from different collections in mongodb - mongodb

Question

Here, I have multiple fields from multiple tables those values needs to compared and need to display desired result.
SQL QUERY:
select pd.service_id,ps.service_id from player pd, service ps where pd.subject_id=ps.subject_id and pd.service_id = ps.service_id
Mongo query:
db.player.aggregate([
{
"$lookup":{
"from":"service",
"localField":"player.subject_id",
"foreignField":"subject_id",
"as":"ps"
}
},
{
"$unwind":"$ps"
},
{
"$match":{
"service_id":{
"$eq": "ps.service_id"
}
}
}
];
sample input records:
player:
[{subject_id:23,service_id:1},{subject_id:76,service_id:9}]
service:
[{subject_id:76,service_id:9},{subject_id:99,service_id:10}]
The match is not working. I have to match service_id's of both collections. Need to get matched records. But not able to see any result. Can anyone please help me to find out the mistake...

Answer 1

In your query, if you want to compare 2 values from the document itself, you need to use $expr operator
{
"$match":{
"$expr":{
"$eq": ["$service_id", "$ps.service_id"]
}
}
}
MongoPlayground
Alternative solution: You need to use Uncorrelated sub-query to "* join" with 2 o more conditions
db.player.aggregate([
{
"$lookup": {
"from": "service",
"let": {
subject_id: "$subject_id",
service_id: "$service_id"
},
"pipeline": [
{
$match: {
$expr: {
$and: [
{
$eq: [
"$$subject_id",
"$subject_id"
]
},
{
$eq: [
"$$service_id",
"$service_id"
]
}
]
}
}
}
],
"as": "ps"
}
},
// Remove non matched results
{
$match: {
"ps.0": {
$exists: true
}
}
},
// Remove temporal "ps" field
{
$addFields: {
"ps": "$$REMOVE"
}
}
])
MongoPlayground