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 "> "
> import Database.MongoDB
> import Database.MongoDB.BSON
> import Data.ByteString.Lazy.UTF8
> c <- connect "localhost" []
> let col = (fromString "test.mr1")
> :{
insertMany c col [
(toBsonDoc [("x", BsonInt32 1),
("tags", toBson ["dog", "cat"])]),
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(toBsonDoc [("x", BsonInt32 2),
("tags", toBson ["cat"])]),
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(toBsonDoc [("x", BsonInt32 3),
("tags", toBson ["mouse", "cat", "doc"])]),
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(toBsonDoc [("x", BsonInt32 4),
("tags", BsonArray [])])
]
:}
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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 = "
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 = "
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 call map_reduce() and iterate over the result collection:
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> mapReduce c col (fromString mapFn) (fromString reduceFn) [] >>= allDocs
[[(Chunk "_id" Empty,BsonString (Chunk "cat" Empty)),
(Chunk "value" Empty,BsonDouble 6.0)],
[(Chunk "_id" Empty,BsonString (Chunk "doc" Empty)),
(Chunk "value" Empty,BsonDouble 1.0)],
[(Chunk "_id" Empty,BsonString (Chunk "dog" Empty)),
(Chunk "value" Empty,BsonDouble 3.0)],
[(Chunk "_id" Empty,BsonString (Chunk "mouse" Empty)),
(Chunk "value" Empty,BsonDouble 2.0)]]
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Advanced Map/Reduce
-------------------
MongoDB returns additional information in the map/reduce results. To
obtain them, use *runMapReduce*:
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> res <- runMapReduce c col (fromString mapFn) (fromString reduceFn) []
> res
[(Chunk "result" Empty, BsonString (Chunk "tmp.mr.mapreduce_1268105512_18" Empty)),
(Chunk "timeMillis" Empty, BsonInt32 90),
(Chunk "counts" Empty,
BsonDoc [(Chunk "input" Empty,BsonInt64 8),
(Chunk "emit" Empty,BsonInt64 12),
(Chunk "output" Empty,BsonInt64 4)]),
(Chunk "ok" Empty,BsonDouble 1.0)]
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You can then obtain the results using *mapReduceResults*:
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> mapReduceResults c (fromString "test") res >>= allDocs
[[(Chunk "_id" Empty,BsonString (Chunk "cat" Empty)),
(Chunk "value" Empty,BsonDouble 6.0)],
[(Chunk "_id" Empty,BsonString (Chunk "doc" Empty)),
(Chunk "value" Empty,BsonDouble 1.0)],
[(Chunk "_id" Empty,BsonString (Chunk "dog" Empty)),
(Chunk "value" Empty,BsonDouble 3.0)],
[(Chunk "_id" Empty,BsonString (Chunk "mouse" Empty)),
(Chunk "value" Empty,BsonDouble 2.0)]]