$sum number of objects on $lookup fields

Question

I am trying to use the $sum in MongoDB to summarize the number of objects in an array. However it only returns 0, even when there are more objects. What am I doing wrong?

{
        $lookup: {
            from: "events",
            let: { user: "$_id" },
            pipeline: [
                { 
                    $match: { 
                        $expr: { 
                            $and: [
                                {$eq: ['$creator', '$$user']},

                            ],
                        },

                    }
                },
                {
                    $match: {
                            'createdAt': { 
                                $gte: moment(a).startOf('day').toDate(),
                                $lte: moment(b).startOf('day').toDate(),
                            }
                        }
                },
                { $project: { _id: 0, leads: 0, bookings: 0, sales: 0 } },
            ],
            as: "events"
        }
    },
    { 
        $addFields: { 
            countEvents: {$sum: "$events"}, 
        }
    }

Answer 1

I think you need to use $size instead of $sum