map/reduce example

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
+-----
+
+To start, we’ll insert some example data which we can perform
+map/reduce queries on:
+
+> $ 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", BsonArray [toBson "dog",
+>                                       toBson "cat"])]),
+>       (toBsonDoc [("x", BsonInt32 2),
+>                   ("tags", BsonArray [toBson "cat"])]),
+>       (toBsonDoc [("x", BsonInt32 3),
+>                   ("tags", BsonArray [toBson "mouse",
+>                                       toBson "cat",
+>                                       toBson "doc"])]),
+>       (toBsonDoc [("x", BsonInt32 4),
+>                   ("tags", BsonArray [])])
+> ]
+> :}
+
+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:
+
+> > :{
+> let mapFn = "
+> function() {\n
+>   this.tags.forEach(function(z) {\n
+>     emit(z, 1);\n
+>   });\n
+> }"
+> :}
+
+The reduce function sums over all of the emitted values for a given
+key:
+
+> > :{
+> 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
+be called iteratively on the results of other reduce steps.
+
+Finally, we call map_reduce() and iterate over the result collection:
+
+> > 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)]]
+
+Advanced Map/Reduce
+-------------------
+
+MongoDB returns additional information in the map/reduce results. To
+obtain them, use *runMapReduce*:
+
+> > 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)]
+
+You can then obtain the results using *mapReduceResults*:
+
+> > 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)]]