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

Question

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

db.getCollection(\'users\').aggregate([
    {
        $project         


        

Answer 1

You need $let operator:

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

Mongo Playground

Answer 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