How to calculate ratios for an additive attribute with mongodb? - mongodb

Question

Using the sample mongodb aggregation collection (http://media.mongodb.org/zips.json), I would like to output the population share of every city in California.
In SQL, it could look like this:
SELECT city, population/SUM(population) as poppct
FROM (
SELECT city, SUM(population) as population
FROM zipcodes
WHERE state='CA'
GROUP BY city
) agg group by state;
This can be done using mongodb map/reduce:
db.runCommand({
mapreduce : "zipcodes"
, out : { inline : 1}
, query : {state: "CA"}
, map : function() {
emit(this.city, this.pop);
cache.totalpop = cache.totalpop || 0;
cache.totalpop += this.pop;
}
, reduce : function(key, values) {
var pop = 0;
values.forEach(function(value) {
if (value && typeof value == 'number' && value > 0) pop += value;
});
return pop;
}
, finalize: function(key, reduced) {
return reduced/cache.totalpop;
}
, scope: { cache: { } }
});
Can this be also achieved using the new aggregation framework (v2.2)? This would require some form of global scope, as in the map/reduce case.
Thanks.

Answer 1

Is this what you're after?
db.zipcodes.remove();
db.zipcodes.insert([
{ city:"birmingham", population:1500000, state:"AL" },
{ city:"London", population:10000, state:"ON" },
{ city:"New York", population:1000, state:"NY" },
{ city:"Denver", population:100, state:"CO" },
{ city:"Los Angeles", population:1000000, state:"CA" },
{ city:"San Francisco", population:2000000, state:"CA" },
]);
db.zipcodes.runCommand("aggregate", { pipeline: [
{ $match: { state: "CA" } }, // WHERE state='CA'
{ $group: {
_id: "$city", // GROUP BY city
population: { $sum: "$population" }, // SUM(population) as population
}},
]});
produces
{
"result" : [
{
"_id" : "San Francisco",
"population" : 2000000
},
{
"_id" : "Los Angeles",
"population" : 1000000
}
],
"ok" : 1
}

Answer 2

you could try:
db.zipcodes.group( { key: { state:1 } ,
reduce: function(curr, result) {
result.total += curr.pop;
result.city.push( { _id: curr.city, pop: curr.pop } ); },
initial: { total: 0, city:[] },
finalize: function (result) {
for (var idx in result.city ) {
result.city[idx].ratio = result.city[idx].pop/result.total;
}
} } )