Aggregate pipeline Match -> Lookup -> Unwind -> Match issue - mongodb

Question

I am puzzled as to why the code below doesn't work. Can anyone explain, please?
For some context: My goal is to get the score associated with an answer option for a survey database where answers are stored in a separate collection from the questions. The questions collection contains an array of answer options, and these answer options have a score.
Running this query:
db.answers.aggregate([
{
$match: {
userId: "abc",
questionId: ObjectId("598be01d4efd70a81c1c5ad4")
}
},
{
$lookup: {
from: "questions",
localField: "questionId",
foreignField: "_id",
as: "question"
}
},
{
$unwind: "$question"
},
{
$unwind: "$question.options"
},
{
$unwind: "$answers"
}
])
I get:
{
"_id" : ObjectId("598e588e0c5e24452c9ee769"),
"userId" : "abc",
"questionId" : ObjectId("598be01d4efd70a81c1c5ad4"),
"answers" : {
"id" : 20
},
"question" : {
"_id" : ObjectId("598be01d4efd70a81c1c5ad4"),
"options" : {
"id" : 10,
"score" : "12"
}
}
}
{
"_id" : ObjectId("598e588e0c5e24452c9ee769"),
"userId" : "abc",
"questionId" : ObjectId("598be01d4efd70a81c1c5ad4"),
"answers" : {
"id" : 20
},
"question" : {
"_id" : ObjectId("598be01d4efd70a81c1c5ad4"),
"options" : {
"id" : 20,
"score" : "4"
}
}
}
All great. If I now add to the original query a match that's supposed to find the answer option having the same id as the answer (e.g. questions.options.id == answers.id), things don't work as I would expect.
The final pipeline is:
db.answers.aggregate([
{
$match: {
userId: "abc",
questionId: ObjectId("598be01d4efd70a81c1c5ad4")
}
},
{
$lookup: {
from: "questions",
localField: "questionId",
foreignField: "_id",
as: "question"
}
},
{
$unwind: "$question"
},
{
$unwind: "$question.options"
},
{
$unwind: "$answers"
},
{
$match: {
"question.options.id": "$answers.id"
}
},
{
$project: {
_id: 0,
score: "$question.options.score"
}
}
])
This returns an empty result. But if I change the RHS of the $match from "$answers.id" to 20, it returns the expected score: 4. I tried everything I could think of, but couldn't get it to work and can't understand why it doesn't work.

Answer 1

I was able to get it to work with the following pipeline:
{
$match: {
userId: "abc",
questionId: ObjectId("598be01d4efd70a81c1c5ad4")
}
},
{
$lookup: {
from: "questions",
localField: "questionId",
foreignField: "_id",
as: "question"
}
},
{
$unwind: "$question"
},
{
$unwind: "$question.options"
},
{
$unwind: "$answers"
},
{
$addFields: {
areEqual: { $eq: [ "$question.options.id", "$answers.id" ] }
}
},
{
$match: {
areEqual: true
}
},
{
$project: {
_id: 0,
score: "$question.options.score"
}
}
I think the reason it didn't work with a direct match is the fact that questions.options.id doesn't actually reference the intended field... I needed to use $questions.options.id which wouldn't work as a LHS of a $match, hence the need to add an extra helper attribute.