似乎没有为nosql中的聚合函数加起来

如何解决似乎没有为nosql中的聚合函数加起来

im是nosql的新手，并且希望提供有关聚合函数的一些指导。我尝试运行此聚合函数，但总的来说，total_new_cases不能在某些月份汇总，当实际上有要累加的new_cases值时显示为null。

db.world.aggregate([    
    {$group:
        {
            _id:{groupbymonth: {$month:"$date"}},total_new_cases: {$sum:"$new_cases"},continent: "$continent"
        }
    },{$project:
        {
            _id: 1,total_new_cases: 1
        }
    },{$sort:{"_id":1}})

以下是输出：

{
    "_id" : {
        "groupbymonth" : 1
    },"total_new_cases" : null
},/* 2 */
{
    "_id" : {
        "groupbymonth" : 2
    },/* 3 */
{
    "_id" : {
        "groupbymonth" : 3
    },/* 4 */
{
    "_id" : {
        "groupbymonth" : 4
    },/* 5 */
{
    "_id" : {
        "groupbymonth" : 5
    },/* 6 */
{
    "_id" : {
        "groupbymonth" : 6
    },/* 7 */
{
    "_id" : {
        "groupbymonth" : 7
    },/* 8 */
{
    "_id" : {
        "groupbymonth" : 8
    },/* 9 */
{
    "_id" : {
        "groupbymonth" : 12
    },"total_new_cases" : 54
}

这是有关变量的某些上下文的集合的摘录：

{
    "_id" : ObjectId("5f4a06e03acddb5eec5561c5"),"iso_code" : "AFG","continent" : "Asia","location" : "Afghanistan","date" : ISODate("2019-12-31T00:00:00.000+08:00"),"total_cases" : 0,"new_cases" : 0,"new_cases_smoothed" : 0,"total_deaths" : 0,"new_deaths" : 0,"new_deaths_smoothed" : 0,"total_cases_per_million" : 0,"new_cases_per_million" : 0,"new_cases_smoothed_per_million" : 0,"total_deaths_per_million" : 0,"new_deaths_per_million" : 0,"new_deaths_smoothed_per_million" : 0,"new_tests" : NaN,"total_tests" : NaN,"total_tests_per_thousand" : 0,"new_tests_per_thousand" : 0,"new_tests_smoothed" : 0,"new_tests_smoothed_per_thousand" : 0,"tests_per_case" : 0,"positive_rate" : 0,"tests_units" : NaN,"stringency_index" : 0,"population" : 38928341,"population_density" : 54.422,"median_age" : 18.6,"aged_65_older" : 2.581,"aged_70_older" : 1.337,"gdp_per_capita" : 1803.987,"extreme_poverty" : "","cardiovasc_death_rate" : 597.029,"diabetes_prevalence" : 9.59,"female_smokers" : NaN,"male_smokers" : NaN,"handwashing_facilities" : 37.746,"hospital_beds_per_thousand" : 0.5,"life_expectancy" : 64.83
}

谢谢。