Sum of two fields based on some conditions - mongodb

Question

These are my documents in Stat collection:
{placeid: 'plaza', guestid: "xxx", logtype: "purchase", value: 12}
{placeid: 'plaza', guestid: "xxx", logtype: "visit", value: 0}
{placeid: 'plaza', guestid: "xxx", logtype: "purchase", value: 17}
{placeid: 'plaza', guestid: "yyy", logtype: "visit", value: 0}
I want to aggregate these documents to get information (passed plaza as argument):
xxx visited: 1, purchases: 2, value of purchases is 29
yyy visited: 1, purchases: 0, value of purchases is 0
This is my approach:
Stat.aggregate(
[
{ $match: { placeid: "plaza" } },
{
$group: {
_id: "$guestid",
totallogs: { $sum: 1 },
totalvalue: { $sum: "$value" },
}
}
]
)
problem here is that this aggregation does not take logtype into consideration.
And I do not know how to improve it. Any help?

Answer 1

You need to use the aggregation framework to $group your documents by "guestid" and use the $sum accumulator operator to return the sum. Of course you also need the $cond operator evaluates the value of "logtype" and returns the value of the "true case".
db.Stat.aggregate([
{ "$match": { "placeid": "plaza" } },
{ "$group": {
"_id": "$guestid",
"visit": {
"$sum": {
"$cond": [
{ "$eq": [ "$logtype", "visit" ] },
1,
0
]
}
},
"purchases": {
"$sum": {
"$cond": [
{ "$eq": [ "$logtype", "purchase" ] },
1,
0
]
}
},
"value_purchase": {
"$sum": {
"$cond": [
{ "$eq": [ "$logtype", "purchase" ] },
"$value",
0
]
}
}
}}
])
which produces:
{ "_id" : "yyy", "visit" : 1, "purchases" : 0, "value_purchase" : 0 }
{ "_id" : "xxx", "visit" : 1, "purchases" : 2, "value_purchase" : 29 }