Perform a join on a collection only WHERE substring matches - arrays

Question

I would like to perform a join on a collection where student name are equal in each collection AND WHERE the last string after "_" in column log's value is what the variable id is.
I got the join to work but the issue is on the match statement. How can I match with a substring of a string on the logs column in the collection that I am about to join?
I can split the log column value into an array like this:
{ $split: [ "$studentInfo.log", "_" ]}
I just need to get the last value after underscore now to match variable id
var id = "123";
dbo.collection("student").aggregate([
{ "$lookup": {
"localField": "name",
"from": "data",
"foreignField": "data.studentName",
"as": "studentInfo"
}
}]).toArray(function(err, results) {
console.log(results);
});
The issue is that student name is not unique so in order to get the join to work correctly we need to join by name AND make sure the characters after underscore match with the variable we have id
student collection
{
"_id" : ObjectId("(Object ID here"),
"name": "Test"
}
data collection
{
"_id" : ObjectId("(Object ID here"),
"studentName": "Test",
"log": "NC_Test_123"
},
{
"_id" : ObjectId("(Object ID here"),
"studentName": "Test",
"log": "FC_Test_444"
}
I need to get NC_Test_123 when the variable I have for id is 123.

Answer 1

You need to define custom lookup condition with both conditions: student name and log field. To get the last value of splitted string you can use $arrayElemAt with index set to -1
db.student.aggregate([
{
$lookup: {
from: "data",
let: { "student_name": "$name" },
pipeline: [
{
$match: {
$expr: {
$and: [
{ $eq: [ "$$student_name", "$studentName" ] },
{ $eq: [ { $arrayElemAt: [ { $split: [ "$log", "_" ]}, -1 ] }, "123" ] }
]
}
}
}
],
as: "studentInfo"
}
},
{
$unwind: "$studentInfo"
}
])