I have the following query which returns user documents that match the search term provided by the requested user:

 // user who made request
 const user = await User.findOne({_id: req.user._id})

 // return users that match the search query provided by requested user
 const usersQuery = await User.aggregate([
        {
            $match: {
                $and: [
                    { username: { $regex: new RegExp(query, "i") } },
                ],
            },
        },
        {
            $project: {
                // This field projects whether the requested user is following this particular user
                isFollowing: { $in: ["$_id", user?.following] },
                username: 1,
                privacyStatus: 1,
            },
        },
    ])

The isFollowing property projects whether the requested user is following the particular user which was returned by the aggregation query.

If the isFollowing property is true I would like to add a $lookup stage to the aggregation which returns all the posts that particular user has created. How would I implement this $lookup stage so that it is only performed if isFollowing is equal to true? Thanks.