Mongo Aggregation How to match an array inside lookup without using $expr - mongodb

Question

I have an aggregation pipeline query (I've removed unnecesary stuff) that work when using $expr but doesn't work without. However, I want to avoid using the $expr for better performance, so the indices will be used. Logically there is a many to many relation here between rule and resource. I want to summarize the cost of the resources per rule. The problem here is to match the resources inside the grouped array without using an expression.
with $expr:
db.collection.aggregate([
{'$group': {'_id': {'rule_id': '$rule_id'}, 'rule_id': {'$first': '$rule_id'}, 'resources_ids': {'$push': '$resource_id'}}},
{'$lookup':
{'from': 'other_collection',
'let': {'resources_ids': '$resources_ids'},
'pipeline': [
{'$match':
{'$expr":
{'$and': [
{'$in':['$resource_id', '$$resources_ids']}
]}
}
},
{'$group': {'_id': {}, 'total_cost': {'$sum': '$cost'}}}], 'as': 'results'}}])
without $expr:
db.collection.aggregate([
{'$group': {'_id': {'rule_id': '$rule_id'}, 'rule_id': {'$first': '$rule_id'}, 'resources_ids': {'$push': '$resource_id'}}},
{'$lookup':
{'from': 'cost_data',
'let': {'resources_ids': '$resources_ids'},
'pipeline': [
{'$match':
{'$and': [
{'resource_id': {'$in': '$$resources_ids'}},
]}
},
{'$group': {'_id': {}, 'total_cost': {'$sum': '$cost'}}}], 'as': 'results'}}])

Answer 1

I think #rickhg12hs' comment gives the right answer. There shouldn't be any need for $expr here. The localField/foreignField syntax will work correctly when the localField is an array without needing to $unwind (or use $expr) as documented here. Therefore the matching component of your $lookup can effectively look like this:
$lookup: {
from: "foreign",
localField: "resources_ids",
foreignField: "resource_id",
as: "result"
}
You can compare the outputs of this syntax above with the more verbose pipeline/$expr version to see that they are the same.
A few other thoughts come to mind. The first is that you can combine the localField/foreignField syntax with the pipeline syntax so that the second $group can still be nested inside of the $lookup. This would make the final version of the $lookup stage structured as follows:
{
$lookup: {
from: "foreign",
localField: "resources_ids",
foreignField: "resource_id",
pipeline: [
{
"$group": {
"_id": {},
"total_cost": {
"$sum": "$cost"
}
}
}
],
as: "result"
}
}
Playground demonstration of that component is here.
The second thing is that using an index to perform the $lookups is important and will likely improve performance, but it may not make this operation "fast". As written, this aggregation will perform a full collection scan to process all of the documents in the source collection. (You will still see that COLLSCAN in the explain output from this source collection even if the index in the other collection is used for the $lookup).
Finally, the index on the other collection should probably be { resource_id: 1, cost: 1 }. This should allow the database to cover the query when doing the $lookups and avoid fetching those documents altogether.
Edit to address this comment:
$expr is not required inside the match. I've done this already. However, here because of the array I am building through the pipeline it doesn't let me use it in the match without an $expr.
This is not correct. Specifically the source of the array isn't relevant here. Whether the array is a field in the source document directly or generated in an earlier pipeline stage doesn't matter to the $lookup stage at all. In fact, it won't even know where that array comes from, just that it is a field in the generated document that is passed to it.
Rather, what you are describing is the behavior of $match itself. From the documentation:
$match takes a document that specifies the query conditions. The query syntax is identical to the read operation query syntax; i.e. $match does not accept raw aggregation expressions. Instead, use a $expr query expression to include aggregation expression in $match.
Said another way, you presently cannot reference any fields from the document (regardless of where they come from or what type and value they have) without $expr.
But that fact should mostly be irrelevant for your use case. You can use the localField/foreignField syntax for this array matching. If you need to match on additional filters then you can also leverage the let/pipeline syntax in the same $lookup. Here is an arbitrary demonstration of that (note the _id: 4 document doesn't match due to the mismatched otherVal).
It is also worth noting that $expr itself does not preclude the usage of indexes in general. One current exception, unfortunately, seems to be with $in (reference). Again though, that shouldn't matter for you if you place that part of the $lookup matching into the localField/foreignField parameters.