I have the following aggregation Query in my application:

const posts = await Post.aggregate([
        { $match: { creator: { $in: req.body.following} } },
        {
            $lookup: {
                from: "users",
                localField: "creator",
                foreignField: "_id",
                as: "creatorDoc",
                pipeline: [
                    {
                        $lookup: {
                            // references stories collection
                            from: "stories",
                            pipeline: [
                                {
                                  // The match below should find the story document with the _id equal to the last id 
                                  // stored inside the stories array attatched to creator documents
 
                                    $match: {
                                        $expr: {
                                            $eq: [
                                                "$_id",
                                                { $arrayElemAt: ["$creatorDoc.stories", -1] },
                                            ],
                                        },
                                    },
                                },
                            ],
                            as: "stories",
                        },
                    },
                ],
            },
        },
        {
            $unwind: "$creatorDoc",
        },
        {
            $project: {
                creator: "$creatorDoc.username",
                // No stories property is returned in the response object
                stories: "$stories",
                isLiked: { $in: [new mongoose.Types.ObjectId(req.user._id), "$likedBy"] },
                createdAt: 1,
            },
        },
    ])

The above query returns posts that meet the $match criteria. I use a $lookup on the creator field of the post in order to return the creator’s user information.

Each creatorDoc has an array called stories which references documents in the Story collection. In the query, I try to $lookup only the last story, by matching a story documents _id to the last id stored in creatorDoc.stories. This should return a stories array containing information regarding only the last story the creator has.

However, inside the result, there is no stories attribute at all. I have also tried to $unwind stories, however this doesn’t accomplish anything. Any ideas why this code is not working? Thanks