Get an array of strings after $lookup instead of an array of objects

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I'm trying to join my members collection with a email collection, which works. But it is showing an array of email objects and I only need an array email addresses. The email address is a field of the email collection. A member can have multiple email addresses. The result should show all the fields from the member collection plus the additional address field.

Members collection

_id: 62692f3503ed123e35a56a12
name: "John",
lastName: "Cena",
// arbitrary number of fields

Emails Collection

_id: 74331a2663ed999e35a33a09
memberId: 62692f3503ed123e35a56a12,
email: "john@john.com"

I tried

db.members.aggregate([{"$lookup" : {from: "emails",localField: "_id",foreignField: "memberId",as: "emails"},

Result

_id: 62692f3503ed123e35a56a12
name: "John",
lastName: "Cena",
emails: [
{
  _id:
  memberId: 62692f3503ed123e35a56a12,
  email: "john@john.com"
},
{...}
]

What I need

_id: 62692f3503ed123e35a56a12
name: "John",
lastName: "Cena",
emails: ["john@john.com", "..."]
// any other fields

How can I do that?

aggregation-framework mongodb-query mongodb aggregation
1 Answers

You can use $map as part of $addFields stage to convert $lookup result:

db.members.aggregate([
    {
        "$lookup" : {
            from: "emails",
            localField: "_id",
            foreignField: "memberId",
            as: "emails"}
    },
    {
        $addFields: {
            emails: { $map: { input: "$emails", as: "$$this.email" } }
        }
    }
]);

Mongo Playground

user_62389770
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