Sum and Count

#ENCODING
import io
import sys
sys.stdout = io.TextIOWrapper(sys.stdout.buffer, encoding='utf-16')
#MONGO
from pymongo import MongoClient
client = MongoClient()
client.progzoo.authenticate('scott','tiger')
db = client['progzoo']
#PRETTY
import pprint
pp = pprint.PrettyPrinter(indent=4)
#CODE
from bson.code import Code

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MapReduce

There are many ways to do this in MongoDB.

count() is a cursor method that takes a query and returns a number equal to the amount of documents that matched the query.
$sum is an aggregation operator availible in the $group stage, that can be used to both sum values and count the number of documents.
mapReduce can produce a sum or a count during the results stage by using JavaScript.

.count()

print(db.world.count({"continent":"Africa"}))

$sum

pp.pprint(list(

   db.world.aggregate([
       {"$group":{
           "_id":"$continent",
           "sum of populations":{"$sum":"$population"},
           "count of countries":{"$sum":1}
       }}
   ])

))

Array.sum()

temp = db.world.map_reduce(

   map=Code("function(){emit(this.continent, this.population)}"),
   reduce=Code("""function(k,v){
                      return Array.sum(v)
                  }
               """),
   out={"inline":1}

)

pp.pprint(temp['results'])