我怎么能得到所有值自定义字段mongoose的总和？

Question

我有架构：

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{
	userName: String
	postCount: Number,
	commentCount: Number,
	abuseCount: Number,
}

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我需要得到总和所有用户计算结果中的所有字段

例如

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{
	"userName": "John"
	"postCount": 5,
	"commentCount": 1,
	"abuseCount": 4,
}, {
	"userName": "Bob"
	"postCount": 11,
	"commentCount": 41,
	"abuseCount": 3,
}, {
	"userName": "Alex"
	"postCount": 2,
	"commentCount": 15,
	"abuseCount": 6,
}

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结果必须是〜

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    {
    	"postCount": 18, (sum all postCount field users)
    	"commentCount": 57,
    	"abuseCount": 10,
    }

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我怎么能这样做？ THX！