MongoTemplate分组平铺字段

```
    @ApiOperation(value = "分组测试", notes = "分组测试", httpMethod = "GET")
    @GetMapping("/groupTest")
    public Result<String> groupTest() {
        List<String> list = List.of("subCategory", "classifyId");
        ProjectionOperation count = Aggregation.project("count");
        for (String s : list) {
            count = count.and("_id." + s).as(s);  // 将 _id 中的字段平铺出来
        }
        count = count.andExclude("_id");
        Aggregation aggregation = Aggregation.newAggregation(
                // $group 阶段: 按 subCategory 和 classifyId 分组，并计算每个分组的数量
                Aggregation.group("subCategory", "classifyId")
                        .count().as("count"),count
        );
        // Step 2: 执行聚合操作
        AggregationResults<JSONObject> results = mongoTemplate.aggregate(aggregation, "wf_design", JSONObject.class);

        // Step 3: 返回结果
        return Result.ofSuccess(results.getMappedResults().toString());
    }

ProjectionOperation：对返回字段进行处理，动态构建时需重新赋值，避免字段映射不上