Spring Data MongoDB: MergeOperation returns the whole collection. Why?

Question

I'm running a MongoDB aggregation where the last stage is a $merge that writes some field to another collection. Basically, this $merge should make an effect only on a single document (this is why we have on: "_id".

Here's how I do it with Java:

var merge = Aggregation.merge().intoCollection("items")
                .on("_id")
                .whenMatched(MergeOperation.WhenDocumentsMatch.mergeDocuments())
                .whenNotMatched(MergeOperation.WhenDocumentsDontMatch.discardDocument())
                .build();

var agg = Aggregation.newAggregation(match, group, merge);
var aggResult = mongoTemplate.aggregate(agg, "prices", Item.class);
  

The aggregation does what it needs and I can see that the requested document has changed, but the problem is that aggregate() returns all the documents in the collection.

This is a major drawback and can't scale well when the collection is large enough.

How can I change my query so it will return only the changed document. Or if not possible, just apply the query without returning anything.

Answer 1

Found the answer and will share for future readers:

mongoTemplate.aggregate(aggregation.withOptions(newAggregationOptions().skipOutput().allowDiskUse(true).build()), "collectionNme", EntityClass.class);