Spring Data + MongoDB无效参考错误

Question

Here is my code (Spring data):

        MatchOperation matchOperation = Aggregation.match(new Criteria("stats.channelId").is(channelId));

        UnwindOperation unwindOperation = Aggregation.unwind("stats");

        GroupOperation groupOperation = Aggregation.group("id", "discordId")
                .push("stats").as("stats");

        AggregationOperation addFields = (AggregationOperationContext aggregationOperationContext) -> {
                DBObject dbObject =
                        new BasicDBObject("allGamesOnChannel",
                                new BasicDBObject("$sum", "$stats.pickupsPlayed"));
                return new BasicDBObject("$addFields", dbObject);
            };

        SortOperation sortOperation = Aggregation.sort(new Sort(Sort.Direction.DESC, "allGamesOnChannel"));
        LimitOperation limitOperation = Aggregation.limit(maxElements);

        ProjectionOperation projectionOperation = Aggregation.project("id", "discordId", "stats");

        Aggregation aggregation = Aggregation.newAggregation(matchOperation,unwindOperation, matchOperation,groupOperation,addFields,
                sortOperation,limitOperation,projectionOperation);

        AggregationResults<UserSummaryChannel> userSummaries = mongoTemplate.aggregate(aggregation, "pickupUser", UserSummaryChannel.class);

I'm getting this exception while executing above:

Caused by: java.lang.IllegalArgumentException: Invalid reference 'allGamesOnChannel'!
    at org.springframework.data.mongodb.core.aggregation.ExposedFieldsAggregationOperationContext.getReference(ExposedFieldsAggregationOperationContext.java:99) ~[spring-data-mongodb-1.10.10.RELEASE.jar:na]
    at org.springframework.data.mongodb.core.aggregation.ExposedFieldsAggregationOperationContext.getReference(ExposedFieldsAggregationOperationContext.java:80) ~[spring-data-mongodb-1.10.10.RELEASE.jar:na]
    at org.springframework.data.mongodb.core.aggregation.SortOperation.toDBObject(SortOperation.java:73) ~[spring-data-mongodb-1.10.10.RELEASE.jar:na]
    at org.springframework.data.mongodb.core.aggregation.AggregationOperationRenderer.toDBObject(AggregationOperationRenderer.java:56) ~[spring-data-mongodb-1.10.10.RELEASE.jar:na]
    at org.springframework.data.mongodb.core.aggregation.Aggregation.toDbObject(Aggregation.java:580) ~[spring-data-mongodb-1.10.10.RELEASE.jar:na]
    at org.springframework.data.mongodb.core.MongoTemplate$BatchAggregationLoader.prepareAggregationCommand(MongoTemplate.java:2603) ~[spring-data-mongodb-1.10.10.RELEASE.jar:na]
    at org.springframework.data.mongodb.core.MongoTemplate$BatchAggregationLoader.aggregate(MongoTemplate.java:2585) ~[spring-data-mongodb-1.10.10.RELEASE.jar:na]
    at org.springframework.data.mongodb.core.MongoTemplate.aggregate(MongoTemplate.java:1570) ~[spring-data-mongodb-1.10.10.RELEASE.jar:na]
    at org.springframework.data.mongodb.core.MongoTemplate.aggregate(MongoTemplate.java:1511) ~[spring-data-mongodb-1.10.10.RELEASE.jar:na]

Executing aggregation without grouping:

Aggregation aggregation = Aggregation.newAggregation(matchOperation,addFields,
            sortOperation,limitOperation,projectionOperation);

gives no error so i assume there is some problem with grouping. Any ideas what i'm doing wrong here? Also what i want to do is to filter list of embedded elements, count sum of one of properties of those elements and sort based on it. Maybe there is similar way to do it?

EDIT So here is my document:

@Document
public class PickupUser {
    @Id
    private String id;
    private String discordId;
    private List<UserModeStats> stats;
    private String name;
}

Embedded object:

@Data
@EqualsAndHashCode(exclude = {"pickupsPlayed", "caps"})
public class UserModeStats {
    public UserModeStats() {}

    private String mode;
    private String channelId;
    private Long pickupsPlayed;
    private Long caps;
}

The output document i would like to create:

@Data
public class UserSummaryChannel {
    public UserSummaryChannel() {}

    private String id;
    private String discordId;
    private List<UserModeStats> stats;
    //private Long allGamesOnChannel;
}

I do not want to include allGamesOnChannel property of UserSummaryChannel (aggregation doesn't work with or without that field).

Answer 1

This is not a real solution but found the other way to achieve my goal. I "implemented" sort operation myself so that insted of this :

SortOperation sortOperation = Aggregation.sort(new Sort(Sort.Direction.DESC, "allGamesOnChannel"));

now i have this:

AggregationOperation sortOperation2 = (AggregationOperationContext aggregationOperationContext) -> {
            DBObject dbObject =
                    new BasicDBObject("allGamesOnChannel", 1);
            return new BasicDBObject("$sort", dbObject);
        };

Rest aggregation operations stay same. No exceptions are being thrown.