MongoDB Aggregate does not properly Sort - mongodb

Question

I've got the following query that works fine. However adding a sort to the aggregation pipeline does not actually sort it. It does change the order of the results but does not properly sort.
Any idea why it refuses to sort the results by date?
db.calls.aggregate({
$match: {
'_id.date': {
$gte: ISODate('2014-04-13T00:00:00.000Z'),
$lte: ISODate('2014-04-20T00:00:00.000Z')
}
}
}, {
$project: {
calls: 1,
year: {
$year: '$_id.date'
},
month: {
$month: '$_id.date'
},
day: {
$dayOfMonth: '$_id.date'
}
}
}, {
$group: {
_id: {
name: "$_id.name",
day: '$day',
year: '$year',
month: '$month'
},
date: {
'$first': '$_id.date'
},
calls: {
'$sum': '$calls'
}
}
}, {
$group: {
_id: '$_id.name',
records: {
$addToSet: {
date: '$date',
calls: '$calls'
}
}
}
}, {
$sort: {
'records.date': 1
}
})
Outputs:
/* 0 */
{
"result" : [
{
"_id" : "a",
"records" : [
{
"date" : ISODate("2014-04-13T00:00:00.000Z"),
"calls" : 522
},
{
"date" : ISODate("2014-04-16T00:00:00.000Z"),
"calls" : 1523
},
{
"date" : ISODate("2014-04-20T00:00:00.000Z"),
"calls" : 57
},
{
"date" : ISODate("2014-04-14T00:00:00.000Z"),
"calls" : 1540
},
{
"date" : ISODate("2014-04-15T00:00:00.000Z"),
"calls" : 1592
},
{
"date" : ISODate("2014-04-17T00:00:00.000Z"),
"calls" : 1466
},
{
"date" : ISODate("2014-04-18T00:00:00.000Z"),
"calls" : 1003
},
{
"date" : ISODate("2014-04-19T00:00:00.000Z"),
"calls" : 623
}
]
},
{
"_id" : "b",
"records" : [
{
"date" : ISODate("2014-04-15T00:00:00.000Z"),
"calls" : 102
},
{
"date" : ISODate("2014-04-17T00:00:00.000Z"),
"calls" : 97
},
{
"date" : ISODate("2014-04-16T00:00:00.000Z"),
"calls" : 116
},
{
"date" : ISODate("2014-04-13T00:00:00.000Z"),
"calls" : 118
},
{
"date" : ISODate("2014-04-14T00:00:00.000Z"),
"calls" : 142
},
{
"date" : ISODate("2014-04-19T00:00:00.000Z"),
"calls" : 68
},
{
"date" : ISODate("2014-04-18T00:00:00.000Z"),
"calls" : 100
},
{
"date" : ISODate("2014-04-20T00:00:00.000Z"),
"calls" : 6
}
]
},
{
"_id" : "c",
"records" : [
{
"date" : ISODate("2014-04-17T00:00:00.000Z"),
"calls" : 137130
},
{
"date" : ISODate("2014-04-14T00:00:00.000Z"),
"calls" : 139497
},
{
"date" : ISODate("2014-04-13T00:00:00.000Z"),
"calls" : 92166
},
{
"date" : ISODate("2014-04-18T00:00:00.000Z"),
"calls" : 123129
},
{
"date" : ISODate("2014-04-15T00:00:00.000Z"),
"calls" : 146390
},
{
"date" : ISODate("2014-04-20T00:00:00.000Z"),
"calls" : 4515
},
{
"date" : ISODate("2014-04-16T00:00:00.000Z"),
"calls" : 141792
},
{
"date" : ISODate("2014-04-19T00:00:00.000Z"),
"calls" : 104847
}
]
}
],
"ok" : 1
}
Document structure:
{
"_id" : {
"date" : ISODate("2014-04-08T12:00:00.000Z"),
"name" : "a"
},
"calls" : 515
}
Fixed by doing a $sort before the last group and switching out $addToSet with $push so the order is preserved.
db.calls.aggregate({
$match: {
'_id.date': {
$gte: ISODate('2014-04-13T00:00:00.000Z'),
$lte: ISODate('2014-04-20T00:00:00.000Z')
}
}
}, {
$project: {
calls: 1,
year: {
$year: '$_id.date'
},
month: {
$month: '$_id.date'
},
day: {
$dayOfMonth: '$_id.date'
}
}
}, {
$group: {
_id: {
name: "$_id.name",
day: '$day',
year: '$year',
month: '$month'
},
date: {
'$first': '$_id.date'
},
calls: {
'$sum': '$calls'
}
}
}, {
$sort: {
date: 1
}
}, {
$group: {
_id: '$_id.name',
records: {
$push: {
date: '$date',
calls: '$calls'
}
}
}
})