How to count occurences of values in a nested array? - mongodb

Question

I've searched but could not find an answer to my problem. I need to count the occurences of the field "nationalCode". I've got a collection with this sample structure in MongoDB:
{
"_id" : ObjectId("5d7519cc6c17d65d4983f048"),
"origin" : "Base1",
"topic" : [
{
"nationalTopic" : {
"nationalCode" : 26
},
"dateTime" : NumberLong(20120927000000)
},
{
"nationalTopic" : {
"nationalCode" : 132
},
"dateTime" : NumberLong(20120927000000)
},
{
"nationalTopic" : {
"nationalCode" : 26
},
"dateTime" : NumberLong(20120927000000)
},
{
"nationalTopic" : {
"nationalCode" : 26
},
"dateTime" : NumberLong(20121005000000)
}
]
}
I've used the following code (I tried many variations of it, but none of them got me the right results):
db.processos.aggregate(
[
{ "$unwind": "$topic" },
{"$match": {"origin": "Base1"}},
{"$group": { "_id": { nationalCode: "$topic.nationalTopic.nationalCode", "count": { "$sum": 1 }} } }
]
)
I'm expecting something like this:
{
"_id" : {
"nationalCode" : 26,
"count" : 3.0
}
}
/* 2 */
{
"_id" : {
"nationalCode" : 132,
"count" : 1.0
}
}

Answer 1

You should extract the count element from the _id.
The following query worked for me.
db.data.aggregate(
[
{ "$unwind": "$topic" },
{"$match": {"origin": "Base1"}},
{"$group": { _id: { "nationalCode": "$topic.nationalTopic.nationalCode" },
"count": {$sum: 1} }
}
]
)

Answer 2

just do it with $project to change your format
do it like this
MongoDB Enterprise >
db.ggg.aggregate(
[
{$unwind:"$topic"},
{"$match": {"origin": "Base1"}},
{"$group": { "_id": { nationalCode: "$topic.nationalTopic.nationalCode"},
"count": { "$sum": 1 } }},
{$project :{"_id.nationalCode":1,"_id.count":"$count"}}
]
)
here it the result !
{ "_id" : { "nationalCode" : 26, "count" : 3 } }
{ "_id" : { "nationalCode" : 132, "count" : 1 } }