Iterate over aggregation on MongoDB with Java

Question

I'm currently having some problems with the usage of java, aggregations and the mongodb.

I have 2 collections in a mongodb.

example collection: person

{
id: 1
name: "Oliver"
companyId: 5
}

example collection: company

{
id: 5
name: asdf
}

Now I want to join those collections by companyId/id (lookup aggregation?) and want to iterate over the result. I dont want to load the whole resultset in to the memory, rather iterate 1 by 1. I think i need some kind of cursor (mongoCursor?).

Im working with Java and Spring. So I have the possibilities to use the Java Mongo Driver (version: 3.7.1) or the Options which provides the Springframework (version 5.0.6).

edit:

In the following example Cursor.hasNext() is always false.

DBObject match = new BasicDBObject("$match",
            new BasicDBObject("companyId", "id"));

    DBObject lookupFields = new BasicDBObject("from", "company");
    lookupFields.put("localField", "companyId");
    lookupFields.put("foreignField", "id");
    lookupFields.put("as", "personWithCompany");
    DBObject lookup = new BasicDBObject("$lookup", lookupFields);

    DBObject projectFields = new BasicDBObject("id", 1);
    projectFields.put("name", 1);
    projectFields.put("companyName", "$company.name);

    List<DBObject> pipeline = Arrays.asList(match, lookup, project);

    Cursor cursor = mongoTemplate.getCollection("person").aggregate(pipeline, AggregationOptions.builder().allowDiskUse(true).build());

    while (cursor.hasNext()) {
        DBObject dbObject = cursor.next();
    }

Answer 1

person.aggregate([
            "$lookup": {
                "from": "company",
                "localField": "companyId",
                "foreignField": "id",
                "as": "PersonDetails"
            },
      {$unwind: "$PersonDetails"}

]);

joining with Person Collection with company