Essentially, I have a User that has an email and a list of subscriptions. I want to create the User with the subscription if the user doesn't exist, if the user exists but doesn't have the subscription I Want to simply add the newSubscription to the Users list of subscriptions. Finally, if both User and subscription already exist then I want to handle the existing subscription in my own way.

await UserDocument.findOneAndUpdate(
            {'$and': [{
                'email': email,
            }, {
                'subscriptions.product': product-id,
            }]},
            {'$setOnInsert': newCustomerWithSubscription,
            '$push': {'email.$.subscription': newSubscription}},  
            {upsert: true, new: false},
            async function(err: CallbackError, doc: UserSchemaType | null) {
                //Code to handle if both exists
            }).exec();

DB Design:

User: {
    email,
    subscriptions: [{
        //sub-info
    }]
}