mongodb - $项目 : Is it possible to access a property of an expression result in just one single stage?

Question

有没有什么办法可以在一个$project中进行以下操作阶段？

db.getCollection('users').aggregate([
    {
        $project : {
            firstEmail : { $arrayElemAt : ["$emails", 0] }
        }
    },
    {
        $project : {
            domain : "$firstEmail.domain"
        }
    }
])

Answer 1

您需要 $let运算符(operator):

db.getCollection('users').aggregate([
    {
        $project : {
            domain : {
                $let: {
                    vars: { firstEmail: { $arrayElemAt : ["$emails", 0] } },
                    in: "$$firstEmail.domain"
                }
            }
        }
    }
])

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