Queries and aggregation - MongoDB in Action - page 95

The reduce function will be passed a key and an array of one or more values. Your job
in writing a reduce function is to make sure that those values are aggregated together
in the desired way and then returned as a single value. Because of map-reduce 's itera-
tive nature, reduce may be invoked more than once, and your code must take this into
account. All this means in practice is that the value returned by the reduce function
must be identical in form to the value emitted by the map function. Look closely and
you'll see that this is the case.
The shell's map-reduce method requires a map and a reduce function as argu-
ments. But this example adds two more. The first is a query filter, which limits the doc-
uments involved in the aggregation to orders made since the beginning of 2010. The
second argument is the name of the output collection:

filter = {purchase_date: {$gte: new Date(2010, 0, 1)}}
db.orders.mapReduce(map, reduce, {query: filter, out: 'totals'})
The results are stored in a collection called totals , and you can query this collec-
tion like you do any other. The following listing displays the results of querying one
of these collections. The _id field holds your grouping key and the year and month,
and the value field references the reduced totals.

Listing 5.3 Querying the map-reduce output collection

> db.totals.find()
{ _id: "1-2011", value: { total: 32002300, items: 59 }}
{ _id: "2-2011", value: { total: 45439500, items: 71 }}
{ _id: "3-2011", value: { total: 54322300, items: 98 }}
{ _id: "4-2011", value: { total: 75534200, items: 115 }}
{ _id: "5-2011", value: { total: 81232100, items: 121 }}
The examples here should give you some sense of MongoDB's aggregation capabili-
ties in practice. In the next section, we'll cover most of the hairy details.

5.4 Aggregation in detail

Here I'll provide some extra details on MongoDB's aggregation functions.

5.4.1 Maxima and minima

You'll commonly need to find min and max values for a given value in a collection.
Databases using SQL provide special min() and max() functions, but MongoDB
doesn't. Instead, you must specify these queries literally. To find a maximum value,
you can sort descending by the desired value and limit the result set to just one docu-
ment. You can get a corresponding minimum by reversing the sort. For example, if
you wanted to find the review with the greatest number of helpful votes, your query
would need to sort by that value and limit by one:

db.reviews.find({}).sort({helpful_votes: -1}).limit(1)
The helpful_votes field in the returned document will contain the maximum value
for that field. To get the minimum value, just reverse the sort order:

db.reviews.find({}).sort({helpful_votes: 1}).limit(1)