spring-data-mongodb aggregation with composite ID - spring

Question

I'm having trouble reading documents from MongoDB using the aggregation framework: I always get null IDs in my results. This only happens for documents that have composite IDs. I tried various versions of spring-data-mongodb (1.10.12, 2.0.7), same result.
Entity definition class
#Document(collection="entities")
public class MyEntity {
static class CompositeKey implements Serializable {
private String stringKey;
private Integer intKey;
public CompositeKey(String stringKey, Integer intKey) {
this.stringKey = stringKey;
this.intKey = intKey;
}
public Integer getIntKey() {
return intKey;
}
public String getStringKey() {
return stringKey;
}
public String toString() {
return "{" + stringKey + " - " + intKey + "}";
}
}
#Id
private CompositeKey id;
private String param;
public MyEntity() {}
public MyEntity(String stringKey, Integer intKey) {
id = new CompositeKey(stringKey, intKey);
}
public CompositeKey getId(){
return id;
}
public void setId(CompositeKey id) {
this.id = id;
}
public String getParam() {
return param;
}
public void setParam(String param) {
this.param = param;
}
}
Testing code
public static void main(String[] args) {
MongoClient client = new MongoClient("127.0.0.1");
SimpleMongoDbFactory factory = new SimpleMongoDbFactory(client, "aggTest");
MongoTemplate mongoTemplate = new MongoTemplate(factory);
MyEntity entity = new MyEntity();
entity.setId(new MyEntity.CompositeKey("one", 1));
entity.setParam("param1");
mongoTemplate.save(entity);
entity = new MyEntity();
entity.setId(new MyEntity.CompositeKey("two", 2));
entity.setParam("param2");
mongoTemplate.save(entity);
Criteria crit = Criteria.where("param").ne("param3");
List<AggregationOperation> aggOpList = new ArrayList<AggregationOperation>();
aggOpList.add(Aggregation.match(crit));
System.out.println("Documents fetched with find: ");
for (MyEntity aggResult : mongoTemplate.find(new Query(crit), MyEntity.class).toArray(new MyEntity[0]))
System.out.println(aggResult.getId() + " - " + aggResult.getParam());
System.out.println("\nDocuments fetched with aggregate: ");
TypedAggregation<MyEntity> aggregation = new TypedAggregation<>(MyEntity.class, aggOpList);
AggregationResults<MyEntity> aggregate = mongoTemplate.aggregate(aggregation, MyEntity.class);
for (MyEntity aggResult : aggregate.getMappedResults())
System.out.println(aggResult.getId() + " - " + aggResult.getParam());
}
Output
Documents fetched with find:
{one - 1} - param1
{two - 2} - param2
Documents fetched with aggregate:
null - param1
null - param2
Debugging into the following method MappingMongoConverter.read(final MongoPersistentEntity entity, final Document bson, final ObjectPath path) I found that in the first case (find method) the documentAccessor variable has the following contents
Document{{_id=Document{{stringKey=one, intKey=1}}, param=param1, _class=MyEntity}}
whereas in the second case (aggregation query) it looks like
Document{{stringKey=one, intKey=1, param=param1, _class=MyEntity}}
The document gets flattened somehow, which makes it impossible for the converter to populate the ID field. I must be doing something wrong, but what?

Answer 1

Spring Data MongoDB lower than 3.x automatically flatten composite id (fields under composite id are unwrapped and place at root object). This is removed in version 3.0 onwards:
https://docs.spring.io/spring-data/mongodb/docs/current/reference/html/#new-features.3.0