Querying the total number of elements in nested arrays - embed documents MongoDB - mongodb

Question

I have documents in my collections like to:
{
_id: 1,
activities: [
{
activity_id: 1,
travel: [
{
point_id: 1,
location: [-76.0,19.1]
},
{
point_id: 2,
location: [-77.0,19.3]
}
]
},
{
activity_id: 2,
travel: [
{
point_id: 3,
location: [-99.3,18.2]
}
]
}
]
},
{
_id: 2,
activities: [
{
activity_id: 3,
travel: [
{
point_id: 4,
location: [-75.0,11.1]
}
]
}
]
}
I can get the total number of activities, as follows:
db.mycollection.aggregate(
{$unwind: "$activities"},
{$project: {count:{$add:1}}},
{$group: {_id: null, number: {$sum: "$count" }}}
)
I get (3 activities):
{ "result" : [ { "_id" : null, "number" : 3 } ], "ok" : 1 }
question: How can I get the total number of elements in all travels?
expected result: 4 elements
these are:
{
point_id: 1,
location: [-76.0,19.1]
},
{
point_id: 2,
location: [-77.0,19.3]
},
{
point_id: 3,
location: [-99.3,18.2]
},
{
point_id: 4,
location: [-75.0,11.1]
}

Answer 1

You can easily transform document by using double $unwind
e.g.
db.collection.aggregate([
{$unwind: "$activities"},
{$unwind: "$activities.travel"},
{$group:{
_id:null,
travel: {$push: {
point_id:"$activities.travel.point_id",
location:"$activities.travel.location"}}
}},
{$project:{_id:0, travel:"$travel"}}
])
This will emit which is very close to your desired output format:
{
"travel" : [
{
"point_id" : 1.0,
"location" : [
-76.0,
19.1
]
},
{
"point_id" : 2.0,
"location" : [
-77.0,
19.3
]
},
{
"point_id" : 3.0,
"location" : [
-99.3,
18.2
]
},
{
"point_id" : 4.0,
"location" : [
-75.0,
11.1
]
}
]
}
Update:
If you just want to know total number of travel documents in whole collection,
try:
db.collection.aggregate([
{$unwind: "$activities"},
{$unwind: "$activities.travel"},
{$group: {_id:0, total:{$sum:1}}}
])
It will print:
{
"_id" : NumberInt(0),
"total" : NumberInt(4)
}
Update 2:
OP wants to filter documents based on some property in aggregation framework. Here is a way to do so:
db.collection.aggregate([
{$unwind: "$activities"},
{$match:{"activities.activity_id":1}},
{$unwind: "$activities.travel"},
{$group: {_id:0, total:{$sum:1}}}
])
It will print (based on sample document):
{ "_id" : 0, "total" : 2 }