mongodb - 如何计算Mongodb聚合中的字段

Question

我有一个包含这样条目的文档

{ 
    "_id": ObjectId("5644c495d0807a1750043237"),
    "siteid": "123456"
    "amount": 1.32
}

有些文件有其他金额，例如。“cashbackAmount”

我想要每个金额字段的 sum 和 count。并非每个文档都包含所有金额字段。

我已经尝试了以下方法

{
    $group: {   
        "_id": "$siteid",
        item2: { "$sum": "$amount" },
        item3: { "$sum": "$totalAmount" },
        item4: { "$sum": "$cashbackAmount" },
        item5: { "$sum": "$unitPrice" },
    }
}  

它给了我总和，但我不知道如何得到每个金额字段出现的次数。

{ "$sum": 1 } 不起作用，因为它为我提供了具有任何一个总计字段的所有文档。

Answer 1

我猜你可能想要这样的东西

db.getCollection('amounts').aggregate([
    {
        $project: {
            siteid: 1,
            amount: 1,
            totalAmount: 1,
            unitPrice: 1,
            cashbackAmount: 1,
            amountPresent: {
                $cond: {
                    if: "$amount",
                    then: 1,
                    else: 0
                }
            },
            totalAmountPresent: {
                $cond: {
                    if: "$totalAmount",
                    then: 1,
                    else: 0
                }
            },
            cashbackAmountPresent: {
                $cond: {
                    if: "$cashbackAmount",
                    then: 1,
                    else: 0
                }
            },
            unitPricePresent: {
                $cond: {
                    if: "$unitPrice",
                    then: 1,
                    else: 0
                }
            }
        }
    },
    {
        $group: {   
            "_id": "$siteid",
            amountSum:   { "$sum": "$amount" },
            amountCount: { "$sum": "$amountPresent" },
            totalAmountSum:   { "$sum": "$totalAmount" },
            totalAmountCount: { "$sum": "$totalAmountPresent" },
            cashbackAmountSum:   { "$sum": "$cashbackAmount" },
            cashbackAmountCount: { "$sum": "$cashbackAmountPresent" },
            unitPriceSum:   { "$sum": "$unitPrice" },
            unitPriceCount: { "$sum": "$unitPricePresent" }
        }
    }
])