mongodb/map-reduce-example.md

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Map/Reduce Example
------------------
This is an example of how to use the mapReduce function to perform
map/reduce style aggregation on your data.
This document has been shamelessly ported from the similar
[pymongo Map/Reduce Example](http://api.mongodb.org/python/1.4%2B/examples/map_reduce.html).
Setup
-----
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To start, we'll insert some example data which we can perform
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map/reduce queries on:
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$ ghci -package mongoDB
GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help
...
Prelude> :set prompt "> "
> :set -XOverloadedStrings
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> import Database.MongoDB
> Right conn <- runNet $ connect $ host "localhost"
> let run act = runNet $ runConn (useDb "test" act) con
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> :{
run $ insertMany "mr1" [
["x" =: 1, "tags" =: ["dog", "cat"]],
["x" =: 2, "tags" =: ["cat"]],
["x" =: 3, "tags" =: ["mouse", "cat", "dog"]],
["x" =: 4, "tags" =: ([] :: [String])]
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]
:}
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Basic Map/Reduce
----------------
Now we'll define our map and reduce functions. In this case we're
performing the same operation as in the MongoDB Map/Reduce
documentation - counting the number of occurrences for each tag in the
tags array, across the entire collection.
Our map function just emits a single (key, 1) pair for each tag in the
array:
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> :{
let mapFn = Javascript [] "
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function() {\n
this.tags.forEach(function(z) {\n
emit(z, 1);\n
});\n
}"
:}
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The reduce function sums over all of the emitted values for a given
key:
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> :{
let reduceFn = Javascript [] "
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function (key, values) {\n
var total = 0;\n
for (var i = 0; i < values.length; i++) {\n
total += values[i];\n
}\n
return total;\n
}"
:}
Note: We can't just return values.length as the reduce function might
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be called iteratively on the results of other reduce steps.
Finally, we run mapReduce and iterate over the result collection:
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> run $ runMR (mapReduce "mr1" mapFn reduceFn) >>= rest
Right (Right [[ _id: "cat", value: 3.0],[ _id: "dog", value: 2.0],[ _id: "mouse", value: 1.0]])
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Advanced Map/Reduce
-------------------
MongoDB returns additional statistics in the map/reduce results. To
obtain them, use *runMR'* instead:
> run $ runMR' (mapReduce "mr1" mapFn reduceFn)
Right (Right [ result: "tmp.mr.mapreduce_1276482643_7", timeMillis: 379, counts: [ input: 4, emit: 6, output: 3], ok: 1.0])
You can then obtain the results from here by quering the result collection yourself. *runMR* (above) does this for you but discards the statistics.