How to use aggregation function mongo db-query

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

E

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);
    });