I am struggling to retrieve comments from a MongoDB collection (using Mongoose) and compute the total number of comments as well as the average rating using the aggregate pipeline. However, if the initial $match query returns no results, the script crashes and displays a 'Cannot read property 'avg' of undefined' error.
var q = Comment.aggregate([
{ $match: {
_post: this,
rating: { $gt: 0 },
}},
{ $group: {
_id: null,
avg: { $avg: "$rating" },
count: { $sum: 1 }
}}
]);
q.exec(function(err, result) {
cb(err, result[0].avg, result[0].count);
});
If I eliminate the _post $match condition, the output shows the correct average and count of all comments. However, if $match does not return any records, an error is thrown. How can this be resolved?