Mongo分组取每组最大的一条记录

使用Spring-data-mongo统计每个班级最高成绩 classIds为List集合

Criteria criteria = Criteria.where("classId").in(classIds);

GroupOperation groupOperation = Aggregation.group("classId").max("score").as("score");

Aggregation agg = Aggregation.newAggregation(ClassScore.class,Aggregation.match(criteria),groupOperation);

AggregationResults<Map> results = mongoTemplate.aggregate(agg,ClassScore.class,Map.class);

 Map<String, Double> classMaxScoreMap = new HashMap<>();

 if(results.getMappedResults() != null && results.getMappedResults().size() != 0){

          for(Map map:results.getMappedResults()){

                       classMaxScoreMap.put(map.get("_id").toString(), Double.valueOf(map.get("score").toString()));

          }

 }

return classMaxScoreMap;

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