Map aggregate query result to correct attributes of POJO

Question

I have a collection named Location which has couple of attributes of which I want to fetch only country code and country name.

When I'm using below aggregation query in Spring Data, all the attributes are getting mapped to the _id attribute of Location.java and not to individual countryCode and countryName attribute of the List. Please help me resolve the same.

@Repository
public class AggregateQueryRepository {
    
@Autowired
ReactiveMongoTemplate reactiveMongoTemplate;
 
public Flux<Location> getAllCountryCodeAndCountry() {
        Aggregation aggregation = newAggregation(group("countryCode", "countryName")
        );
        return reactiveMongoTemplate.aggregate(aggregation, "location", Location.class);
    }
}

Currently each object of the result looks like below:

{ "_id": "{\"countryCode\": \"IN\", \"countryName\": \"India\"}" }

I want to map it to something like below:

{
{ "_id": null,
"countryCode": "ÏN"
"countryName": "India"
},
....
.....
}

Answer 1

If you must have it separately, you can consider using $first aggregation accumulator:

Aggregation aggregation = newAggregation(
    group("countryCode", "countryName")
        .first("countryCode").as("countryCode")
        .first("countryName").as("countryName")
);

return reactiveMongoTemplate.aggregate(aggregation, "location", Location.class);

...or alternatively $replaceRoot aggregation stage:

Aggregation aggregation = newAggregation(
    group("countryCode", "countryName"),
    replaceRoot("_id")
);

return reactiveMongoTemplate.aggregate(aggregation, "location", Location.class);