$lookup on ObjectId's in an array

Question

What\'s the syntax for doing a $lookup on a field that is an array of ObjectIds rather than just a single ObjectId?

Example Order Document:

{
  _id:          


        

Answer 1

I have to disagree, we can make $lookup work with IDs array if we preface it with $match stage.

// replace IDs array with lookup results
db.products.aggregate([
    { $match: { products : { $exists: true } } },
    {
        $lookup: {
            from: "products",
            localField: "products",
            foreignField: "_id",
            as: "productObjects"
        }
    }
])

It becomes more complicated if we want to pass the lookup result to a pipeline. But then again there's a way to do so (already suggested by @user12164):

// replace IDs array with lookup results passed to pipeline
db.products.aggregate([
    { $match: { products : { $exists: true } } },
    {
        $lookup: {
            from: "products",
             let: { products: "$products"},
             pipeline: [
                 { $match: { $expr: {$in: ["$_id", "$$products"] } } },
                 { $project: {_id: 0} } // suppress _id
             ],
            as: "productObjects"
        }
    }
])