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

问题

Is there any way to carry out the following operation in just one $project stage?

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

回答1:

You need $let operator:

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

Mongo Playground

回答2:

When working with array of objects, you can automatically get array of objects's properties with the dot notation. So the following will perfectly work :

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

Mongo playground

来源：https://stackoverflow.com/questions/60287936/project-is-it-possible-to-access-a-property-of-an-expression-result-in-just-on