How to sum aggregation of NumberLong values in mongo?
(如何对Mongo中的NumberLong值的聚合求和？)
Usual sum operator returns 0 for cases with such type.
(对于这种类型的情况，通常的总和运算符将返回0。)
Maybe conversion required to achieve this.
(可能需要转换才能实现此目的。)
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