MapReduce in PyMongo

My Mongo collection: Impressions

has documents in the following format: -

   {
        _uid: 10,
        "impressions": [
            {
                "pos": 6,
                "id": 123,
                "service": "furniture"
            },
            {
                "pos": 0,
                "id": 128,
                "service": "electronics"
            },
            {
                "pos": 2,
                "id": 127,
                "service": "furniture"
            },
            {
                "pos": 2,
                "id": 125,
                "service": "electronics"
            },
            {
                "pos": 10,
                "id": 124,
                "service": "electronics"
            }
        ]
      },
     {
        _uid: 11,
        "impressions": [
            {
                "pos": 1,
                "id": 124,
                "service": "furniture"
            },
            {
                "pos": 10,
                "id": 124,
                "service": "electronics"
            },
            {
                "pos": 1,
                "id": 123,
                "service": "furniture"
            },
            {
                "pos": 21,
                "id": 122,
                "service": "furniture"
            },
            {
                "pos": 3,
                "id": 125,
                "service": "electronics"
            },
            {
                "pos": 10,
                "id": 121,
                "service": "electronics"
            }
            ]
         },
            .
            .
            .
            .
            .

      

        
        
        
      

    

Each of the documents in the collection has a key "impressions"

, which is an array of dictionaries. Each dictionary "id"

has an object identifier, "service"

a service type, and "pos"

a position position in the search results. My goal is to find out the number of impressions for each "id"

in each category. So for the above data for "service"

== "furniture"

, I want this to be like my aggregation results: -

[
{"id": 123,"impressions_count":2},
{"id": 127,"impressions_count":1},
{"id": 124,"impressions_count":1},
{"id": 122,"impressions_count":1}
]

      

        
        
        
      

    

I tried to aggregate on "id" using MAPREDUCE with the following function in a python script

def fetch_impressions():
    try:
        imp_collection = get_mongo_connection('Impressions')
        map = Code("""
                function(){
                    for( x in this.impressions){
                        var flat_id = x['id'];
                        var service_type = x['service']
                        emit(parseInt(flat_id),1);
                        }
                    };
                """)

                        """)
        reduce = Code("""
                        function(a,b){
                            return Array.sum(b);
                            };
                        """)

        results = imp_collection.map_reduce(map, reduce, 'aggregation_result')
        return results
    except Exception as e:
        raise Exception(e)

      

        
        
        
      

    

But I am getting results as "No", possibly due to a faulty card function. I'm new to Javascript and Mongo, kindly help!