Question

I’m trying to calculate all documents where a certain field exists (in this case it’s “country” field) with Map+Reduce, and the only solution that worked for me is this:

mapper = Code("""
function () {
if (typeof this.country != 'undefined') { 
    var key = 1
    emit(key, {count: 1})
    };
};
""")

I’m not really interested in keys, just if field exists, so I just passed 1.
But I’m sure that’s wrong.

reducer = Code("""
function (key, values) {
    var sum = 0;
    values.forEach(function (value) {
        sum += value['count'];
    });
    return {count: sum};
};
""")

And then calling map_reduce:

results = dbHandle.cards.map_reduce(mapReduce.mapper, mapReduce.reducer, "resultsMR")
for doc in results.find():
    print "Found %s documents." % int(doc.get('value').get('count'))

Also I’m thinking on how to get the amount of docs where their creation date is > than other date, should I use a “query” option in map_reduce function?

query = {"foundationDate":{"$gt":datetime.datetime(2012, 1, 1, 00, 00, 00)}}

Thank you 🙂

Answer 1

Per Chien-Wei Huang comment, why not use the in built functionality e.g.

db.somName.find({"country":{"$exists":True}}).count() 

Else if you want some token map-reduce code you cold simply mimic the count functionality, via the group() function e.g.

db.somName.group(
     { cond: { "country" : {$exists : true} }
      , key: { }
      , initial: {count : 0}
      , reduce: function(doc, out){ out.count++}
     }
  );

Note: If you also want a count by country value, then stick the following in the key:
, key { “country” : 1 }