Your ids variable will be constructed of “strings”, and not ObjectId values.
Mongoose “autocasts” string values for ObjectId into their correct type in regular queries, but this does not happen in the aggregation pipeline, as in described in issue #1399.
Instead you must do the correct casting to type manually:
ids = ids.map(function(el) { return mongoose.Types.ObjectId(el) })
Then you can use them in your pipeline stage:
{ "$match": { "_id": { "$in": ids } } }
The reason is because aggregation pipelines “typically” alter the document structure, and therefore mongoose makes no presumption that the “schema” applies to the document in any given pipeline stage.
It is arguable that the “first” pipeline stage when it is a $match stage should do this, since indeed the document is not altered. But right now this is not how it happens.
Any values that may possibly be “strings” or at least not the correct BSON type need to be manually cast in order to match.