Aggregate $match _id $eq $toObjectId not working

[steevej (Steeve Juneau)]

See on a related topic

$lookup foreignField is ObjectId

That is why it is better to keep your ObjectId as ObjectId rather than strings. In addition an ObjectId probably takes less space than it’s string representation. But your you’re in luck, see the following: The drawbacks is that each and every string must be converted. Most likely no index can be used because of the conversion. You might need an alternate form of $lookup because you might need to use let and may be enclosed the conversion into an $expr.

and

Aggregation between 2 tables with join using field that is array of strings

Your problem is that you keep your document ids as string in your batches collection. This means that every time you lookup you must convert your string ids into ObjectId or the other way around. This means you will not be able to use the index on the converted values. Since it is best to keep ObjectId as ObjectId, the first step is to make sure DocumentIDs holds the ID as ObjectId. Recent $lookup is more powerfull than the one you shown. See https://docs.mongodb.com/manual/reference/operat…