如何使用Spring Data MongoDB计算按另一个字段分组的字段和

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.mongodb.core.MongoTemplate;
import org.springframework.data.mongodb.core.aggregation.Aggregation;
import org.springframework.data.mongodb.core.aggregation.AggregationResults;
import org.springframework.data.mongodb.core.aggregation.GroupOperation;
import org.springframework.data.mongodb.core.aggregation.MatchOperation;
import org.springframework.data.mongodb.core.aggregation.ProjectionOperation;
import org.springframework.data.mongodb.core.query.Criteria;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class SalesService {

    @Autowired
    private MongoTemplate mongoTemplate;

    public List<ProductSalesTotal> calculateTotalQuantityByProduct() {
        // 分组操作：按product字段分组，并计算每个组的quantity总和
        GroupOperation groupByProduct = Aggregation.group("product")
                .sum("quantity").as("totalQuantity");

        // 构建完整的聚合管道
        Aggregation aggregation = Aggregation.newAggregation(groupByProduct);

        // 执行聚合查询并获取结果
        AggregationResults<ProductSalesTotal> results = mongoTemplate.aggregate(
                aggregation, "sales", ProductSalesTotal.class);

        return results.getMappedResults();
    }
}

// 结果封装类
class ProductSalesTotal {
    private String product;
    private int totalQuantity;

    // getters and setters
}

MatchOperation matchStage = Aggregation.match(Criteria.where("quantity").gt(0));
Aggregation aggregation = Aggregation.newAggregation(matchStage, groupByProduct);