Mongoose aggregate get total numbers of members in a group model - node.js

Question

I have a model like this :
var field = {
createdAt: {type: Date, default: Date.now()},
updatedAt: {type: Date, default: Date.now()},
pic : {type: String, default: 'http://lorempixel.com/400/400/abstract/', writable: true},
thumb : {type: String, default: 'http://lorempixel.com/100/100/abstract/', writable: true},
name: {type: String},
description: {type: String},
isPublic: {type: Boolean, default: true},
members: [{type: Schema.Types.ObjectId, ref: 'Member'}],
}
Im using this code below to get the total count of the Member's ID in members field.
Group.aggregate([
{$match: {_id:req.params.group_id}},
{$group: {
_id: '$members',
count: {$sum: 1}
}}
], function (err, count) {
res.send(count);
});
But it returns and empty array [] how do I get the proper count?
Thanks.

Answer 1

Mongoose does not "autocast" to OjectId or any other schema type within the aggregation pipeline. So your operation returns nothing since it matches nothing.
Also, that would not be the approach to counting members of an array.
Better to use the $size option instead to count array members per document:
Group.aggregate([
{ "$match": { _id: ObjectId(req.params.group_id) }},
{ "$group": {
"_id": null,
"count": { "$sum": { "$size": "$members" } }
}}
], function (err, count) {
res.send(count);
});
And $group with the _id for the appropriate roll-up, or null as used here for all items in the collection.
If you wanted the "count of each distinct member" then you would process with $unwind across document(s) instead:
Group.aggregate([
{ "$match": { _id: ObjectId(req.params.group_id) }},
{ "$unwind": "$members" },
{ "$group": {
"_id": "$members",
"count": { "$sum": 1 }
}}
], function (err, count) {
res.send(count);
});