total of all groups totals using mongodb - mongodb

Question

i did this Aggregate pipeline , and i want add a field contains the Global Total of all groups total.
{ "$match": query },
{ "$sort": cursor.sort },
{ "$group": {
_id: { key:"$paymentFromId"},
items: {
$push: {
_id:"$_id",
value:"$value",
transaction:"$transaction",
paymentMethod:"$paymentMethod",
createdAt:"$createdAt",
...
}
},
count:{$sum:1},
total:{$sum:"$value"}
}}
{
//i want to get
...project groups , goupsTotal , groupsCount
}
,{
"$skip":cursor.skip
},{
"$limit":cursor.limit
},
])

Answer 1

you need to use $facet (avaialble from MongoDB 3.4) to apply multiple pipelines on the same set of docs
first pipeline: skip and limit docs
second pipeline: calculate total of all groups
{ "$match": query },
{ "$sort": cursor.sort },
{ "$group": {
_id: { key:"$paymentFromId"},
items: {
$push: "$$CURRENT"
},
count:{$sum:1},
total:{$sum:"$value"}
}
},
{
$facet: {
docs: [
{ $skip:cursor.skip },
{ $limit:cursor.limit }
],
overall: [
{$group: {
_id: null,
groupsTotal: {$sum: '$total'},
groupsCount:{ $sum: '$count'}
}
}
]
}
the final output will be
{
docs: [ .... ], // array of {_id, items, count, total}
overall: { } // object with properties groupsTotal, groupsCount
}
PS: I've replaced the items in the third pipe stage with $$CURRENT which adds the whole document for the sake of simplicity, if you need custom properties then specify them.

Answer 2

i did it in this way , project the $group result in new field doc and $sum the sub totals.
{
$project: {
"doc": {
"_id": "$_id",
"total": "$total",
"items":"$items",
"count":"$count"
}
}
},{
$group: {
"_id": null,
"globalTotal": {
$sum: "$doc.total"
},
"result": {
$push: "$doc"
}
}
},
{
$project: {
"result": 1,
//paging "result": {$slice: [ "$result", cursor.skip,cursor.limit ] },
"_id": 0,
"globalTotal": 1
}
}
the output
[
{
globalTotal: 121500,
result: [ [group1], [group2], [group3], ... ]
}
]