MongoDB lookup when foreign field is an array

Question

I've searched the internet and StackOverflow, but I cannot find the answer or even the question.

I have two collections, reports and users. I want my query to return all reports and indicate if the specified user has that report as a favorite in their array.

Reports Collection

{ _id: 1, name:"Report One"}
{ _id: 2, name:"Report Two"}
{ _id: 3, name:"Report Three"}

Users Collection

{_id: 1, name:"Mike", favorites: [1,3]}
{_id: 2, name:"Tim", favorites: [2,3]}

Desired Result for users.name="Mike"

{ _id: 1, name:"Report One", favorite: true}
{ _id: 2, name:"Report Two", favorite: false}
{ _id: 3, name:"Report Three", favorite: true}

All of the answers I can find use $unwind on the local (reports) field, but in this case the local field isn't an array. The foreign field is the array.

How can I unwind the foreign field? Is there a better way to do this?

I saw online that someone suggested making another collection favorites that would contain:

{ _id: 1, userId: 1, reportId: 1 }
{ _id: 2, userId: 1, reportId: 3 }
{ _id: 3, userId: 2, reportId: 2 }
{ _id: 4, userId: 2, reportId: 3 }

This method seems like it should be unnessesary. It should be simple to join onto an ID in a foreign array, right?

Answer 1

You can use $lookup with custom pipeline which will give you 0 or 1 result and then use $size to convert an array to single boolean value:

db.reports.aggregate([
    {
        $lookup: {
            from: "users",
            let: { report_id: "$_id" },
            pipeline: [
                {
                    $match: {
                        $expr: {
                            $and: [
                                { $eq: [ "$name", "Mike" ] },
                                { $in: [ "$$report_id", "$favorites" ] }
                            ]
                        }
                    }
                }
            ],
            as: "users"
        }
    },
    {
        $project: {
            _id: 1,
            name: 1,
            favorite: { $eq: [ { $size: "$users" }, 1 ] }
        }
    }
])

Alternatively if you need to use MongoDB version lower than 3.6 you can use regular $lookup and then use $filter to get only those users where name is Mike:

db.reports.aggregate([
    {
        $lookup: {
            from: "users",
            localField: "_id",
            foreignField: "favorites",
            as: "users"
        }
    },
    {
        $project: {
            _id: 1,
            name: 1,
            favorite: { $eq: [ { $size: { $filter: { input: "$users", as: "u", cond: { $eq: [ "$$u.name", "Mike" ] } } } }, 1 ] }
        }
    }
])