MongoDB聚合错误：参考无效

Question

你好，我在Collection上运行聚合操作时遇到此错误。我在FarmerCropDataLog集合中有1000个条目。

{
    "_id" : ObjectId("5c7a590350d039fdc86e4e26"),
    "cropData" : {
        "cropName" : "PADDY",
        "crop" : "PADDY",
        "cropAcres" : 60.0,
        "cropYield" : 21.0,
        "cropPrice" : 6183.0
    },
    "creationTime" : "1551521509924",
    "villageId" : "581edb47e4b0b5b1ebbfe753"
}

我想获取按村庄分组的cropPrice的平均价值。所以我写了下面的代码。

public void getComparisonSheet(GetComparisonSheetRequest getComparisonSheet, BasicResponse response,
            String requestFarmerId) {

        Farmer farmer = this.getFarmerById(requestFarmerId);

        // get logs before this year..
        String villageId = farmer.getVillageId();

        MatchOperation matchOperation = Aggregation.match(Criteria.where(FarmerCropDataLog.Constants.CROP_LOG)
                .elemMatch(Criteria.where(CropData.Constants.CROP).is(getComparisonSheet.getCrop())
                        .and(FarmerCropDataLog.Constants.VILLAGE_ID).is(villageId)
                        .and(FarmerCropDataLog.Constants.CREATION_TIME).gt(LAST_YEAR)));

        GroupOperation groupOperation = Aggregation.group(FarmerCropDataLog.Constants.VILLAGE_ID);
        groupOperation.avg(CropData.Constants.CROP_PRICE).as(GetComparisonSheetResponse.Constants.AVERAGE);
        groupOperation.max(CropData.Constants.CROP_PRICE).as(GetComparisonSheetResponse.Constants.MAX_INCOME);

        LimitOperation limitOperation = new LimitOperation(1);

        Fields fields = Fields.fields(GetComparisonSheetResponse.Constants.AVERAGE,GetComparisonSheetResponse.Constants.MAX_INCOME);
        ProjectionOperation projectionOperation = Aggregation.project(fields);

        Aggregation aggregation = Aggregation.newAggregation(matchOperation, groupOperation, projectionOperation,limitOperation);
        AggregationResults<GetComparisonSheetResponse>  aggregationResults = farmerCropDataLogDAO.runAggregation(aggregation, FarmerCropDataLog.class, GetComparisonSheetResponse.class);

        List<GetComparisonSheetResponse> comparisonSheetResponses = aggregationResults.getMappedResults();
        Float AverageIncome = comparisonSheetResponses.get(0).getAverageIncome();
        Float MaxIncome = comparisonSheetResponses.get(0).getMaxIncome();

        GetComparisonSheetResponse comparisonSheetResponse = new GetComparisonSheetResponse();
        comparisonSheetResponse.setAverageIncome(AverageIncome);
        comparisonSheetResponse.setMaxIncome(MaxIncome);

        response.setResponse(comparisonSheetResponse);
        response.setSuccess(true);
    }

这是我遇到的错误：无效的参考'averageIncome'！

这是我的FarmerCropDataLog类的样子。

public class FarmerCropDataLog extends AbstractEntity {
    private String farmerId;
    private CropData cropData;
    private String villageId;
}

这是我的CropData类。

public class CropData {

    private String cropName;
    private Crop crop;
    private Float cropAcres;
    private Float cropYield;
    private Float cropPrice;
}