Aggregate results, sum only if condition is true - node.js

Question

I'm trying to get a sum of all values of $revenue, and a count of only where $user is equal to the user param I pass when calling this function.
this.aggregate([
{ $match: { createdAt: { $gte: start, $lte: end }, 'status.verified': true } },
{
$group: {
_id: null,
balance: {
$sum: "$revenue"
},
count: {
$cond: {
if: { $eq: [ "$user", user ] },
then: { $sum: 1 },
else: { $sum: 0 }
}
}
}
}
], next);
I'm expecting the data to look like this:
[ { _id: null, balance: 1287, count: 10 ] }
Where balance is the sum of all revenue fields in the match query, and count is the count of that users contributions to the data.
It works fine if I sum the count unconditionally (e.g. like this)
this.aggregate([
{ $match: { createdAt: { $gte: start, $lte: end }, 'status.verified': true } },
{
$group: {
_id: null,
balance: {
$sum: "$revenue"
},
count: { $sum: 1 }
}
}
], next);
Which suggests the error is with my conditional sum. The error thrown by MongoDB is
TypeError: Cannot read property '0' of undefined
at redacted:10:20
at redacted/node_modules/mongoose/lib/aggregate.js:529:13
My schema is
var schema = new Schema({
user: { type: Schema.Types.ObjectId, ref: 'User' },
status: {
verified: { type: Boolean, default: false },
completed: { type: Boolean, default: false },
canceled: { type: Boolean, default: false },
refused: { type: Boolean, default: false }
},
meta: { type: Schema.Types.Mixed, default: {} },
revenue: { type: Number, default: 0 }
});
Note: the createdAt value used in $match is inserted automatically by a plugin.

Answer 1

The $cond operator should essentially be an expression of the $sum operator, like the following:
this.aggregate([
{ "$match": {
"createdAt": { "$gte": start, "$lte": end },
"status.verified": true
} },
{
"$group": {
"_id": null,
"balance": { "$sum": "$revenue" },
"count": {
"$sum": {
"$cond": [
{ "$eq": [ "$user", user ] },
1, 0
]
}
}
}
}
], next);