How to use aggregation function mongo db-query - mongodb

Question

I am new in MongoDB and I would like to use the aggregation function where I want to check type == topic and get the following output
Expected output
[
{
conceptName : 59d98cfd1c5edc24e4024d00
totalCount : 2
},
{
conceptName : 59d98cfd1c5edc24e4024d03
totalCount : 1
}
]
Sample input db.GroupContents
{
"_id" : "5a0948bb1c5edc7a5000521a",
"type" : "topic",
"groupID" : "5a0948bb1c5edc7a5000521a",
"pedagogyID" : "59d98cfa1c5edc24e40249a3",
}
Sample input db.PedagogyNodes
{
"_id" : "59d98cfa1c5edc24e40249a3",
"latestVersion" : "59d98cfa1c5edc24e402497f_1",
"createdAt" : "2017-10-08 04:27:06",
"updatedAt" : "2017-10-08 04:27:06"
}
Sample input db.PedagogyVersions
{
"_id" : "59d98cfa1c5edc24e402497f_1",
"type" : "topic",
"contentNodes" : {
"LearningNodes" : [
"59d98cfd1c5edc24e4024d00",
"59d98cfd1c5edc24e4024d03",
"59d98cfd1c5edc24e4024d00",
]
},
"createdAt" : "2017-10-08 04:27:06",
"updatedAt" : "2017-10-08 04:27:06"
}
What I have tried so far
var groupID = "5a0948bb1c5edc7a5000521a"; // Step 1
var records;
var pnDoc;
var pvDoc;
db.GroupContents.find({groupID : groupID}).forEach(function (doc){ // Step 2
var pedagogyID = doc.pedagogyID;
var records = db.getSiblingDB('PedagogyService');
records.PedagogyNodes.find({_id : pedagogyID}).forEach(function (pnDoc) { // Step 3
var latestVersion = pnDoc.latestVersion;
// addded aggregate function here
records.PedagogyVersions.aggregate([
{
$match:{_id:latestVersion} // Step 4
},
{
$unwind:"$contentNodes.LearningNodes"
},
{
$group:
{
_id:"$contentNodes.LearningNodes",
count:{$sum:1}
}
}
])
})
});
I am unable to write db query based on my expected answer, please help.
Understand my requirement
Step : 1 => I am passing `groupID = 5a0948bb1c5edc7a5000521a`
Step : 2 => we have to check from GroupContents where groupID = groupID then we have to take `pedagogyID`
Step : 3 => we have to check from PedagogyNodes where _id = pedagogyID then we have to take `latestVersion`
Step : 4 => we have to check from PedagogyVersions where _id = latestVersion then we have to take `contentNodes->LearningNodes`
Step : 5 => Finally we have to do the aggregation then we have display the result

Answer 1

Try to unwind the LearningNodes array and then count them by grouping them together
db.PedagogyNodes.aggregate([
{
$unwind:"$contentNodes.LearningNodes"
},
{
$group:
{
_id:"$contentNodes.LearningNodes",
count:{$sum:1}
}
}
])
In case you need to do any matches you can use the $match stage
db.PedagogyNodes.aggregate([
{
$match:{type:"topic"}
},
{
$unwind:"$contentNodes.LearningNodes"
},
{
$group:
{
_id:"$contentNodes.LearningNodes",
count:{$sum:1}
}
}
])
Answering the edited question =>
You were not able to view the output on the console since mongoshell does not print script output on the screen. To do this, do the following:
var result = records.PedagogyVersions.aggregate([......]);
result.forEach(function(resultDoc){
print(tojson(resultDoc))
})

Answer 2

To see the result of your aggregation you have to pass the callback to be executed as parameter.
records.PedagogyVersions.aggregate([
{
$match:{_id:latestVersion} // Step 4
},
{
$unwind:"$contentNodes.LearningNodes"
},
{
$group:
{
_id:"$contentNodes.LearningNodes",
count:{$sum:1}
}
}
], function(err, results) {
console.log(results);
});