fix map-reduce-example formatting
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Scott R. Parish
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1 changed files with 71 additions and 51 deletions
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@ -10,33 +10,33 @@ This document has been shamelessly ported from the similar
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Setup
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-----
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To start, we’ll insert some example data which we can perform
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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
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> GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help
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> ...
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> Prelude> :set prompt "> "
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> > import Database.MongoDB
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> > import Database.MongoDB.BSON
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> > import Data.ByteString.Lazy.UTF8
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> > c <- connect "localhost" []
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> > let col = (fromString "test.mr1")
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> > :{
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> insertMany c col [
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> (toBsonDoc [("x", BsonInt32 1),
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> ("tags", BsonArray [toBson "dog",
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> toBson "cat"])]),
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> (toBsonDoc [("x", BsonInt32 2),
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> ("tags", BsonArray [toBson "cat"])]),
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> (toBsonDoc [("x", BsonInt32 3),
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> ("tags", BsonArray [toBson "mouse",
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> toBson "cat",
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> toBson "doc"])]),
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> (toBsonDoc [("x", BsonInt32 4),
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> ("tags", BsonArray [])])
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> ]
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> :}
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$ ghci -package mongoDB
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GHCi, version 6.12.1: http://www.haskell.org/ghc/ :? for help
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...
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Prelude> :set prompt "> "
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> import Database.MongoDB
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> import Database.MongoDB.BSON
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> import Data.ByteString.Lazy.UTF8
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> c <- connect "localhost" []
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> let col = (fromString "test.mr1")
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> :{
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insertMany c col [
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(toBsonDoc [("x", BsonInt32 1),
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("tags", BsonArray [toBson "dog",
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toBson "cat"])]),
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(toBsonDoc [("x", BsonInt32 2),
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("tags", BsonArray [toBson "cat"])]),
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(toBsonDoc [("x", BsonInt32 3),
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("tags", BsonArray [toBson "mouse",
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toBson "cat",
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toBson "doc"])]),
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(toBsonDoc [("x", BsonInt32 4),
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("tags", BsonArray [])])
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]
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:}
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Basic Map/Reduce
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----------------
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@ -49,36 +49,43 @@ tags array, across the entire collection.
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Our map function just emits a single (key, 1) pair for each tag in the
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array:
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> > :{
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> let mapFn = "
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> function() {\n
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> this.tags.forEach(function(z) {\n
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> emit(z, 1);\n
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> });\n
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> }"
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> :}
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> :{
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let mapFn = "
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function() {\n
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this.tags.forEach(function(z) {\n
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emit(z, 1);\n
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});\n
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}"
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:}
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The reduce function sums over all of the emitted values for a given
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key:
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> > :{
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> let reduceFn = "
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> function (key, values) {\n
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> var total = 0;\n
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> for (var i = 0; i < values.length; i++) {\n
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> total += values[i];\n
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> }\n
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> return total;\n
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> }"
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> :}
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> :{
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let reduceFn = "
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function (key, values) {\n
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var total = 0;\n
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for (var i = 0; i < values.length; i++) {\n
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total += values[i];\n
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}\n
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return total;\n
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}"
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:}
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Note: We can’t just return values.length as the reduce function might
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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.
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Finally, we call map_reduce() and iterate over the result collection:
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> > mapReduce c col (fromString mapFn) (fromString reduceFn) [] >>= allDocs
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> [[(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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> mapReduce c col (fromString mapFn) (fromString reduceFn) [] >>= allDocs
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[[(Chunk "_id" Empty,BsonString (Chunk "cat" Empty)),
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(Chunk "value" Empty,BsonDouble 6.0)],
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[(Chunk "_id" Empty,BsonString (Chunk "doc" Empty)),
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(Chunk "value" Empty,BsonDouble 1.0)],
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[(Chunk "_id" Empty,BsonString (Chunk "dog" Empty)),
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(Chunk "value" Empty,BsonDouble 3.0)],
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[(Chunk "_id" Empty,BsonString (Chunk "mouse" Empty)),
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(Chunk "value" Empty,BsonDouble 2.0)]]
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Advanced Map/Reduce
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-------------------
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@ -86,11 +93,24 @@ Advanced Map/Reduce
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MongoDB returns additional information in the map/reduce results. To
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obtain them, use *runMapReduce*:
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> > res <- runMapReduce c col (fromString mapFn) (fromString reduceFn) []
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> > res
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> [(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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> res <- runMapReduce c col (fromString mapFn) (fromString reduceFn) []
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> res
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[(Chunk "result" Empty, BsonString (Chunk "tmp.mr.mapreduce_1268105512_18" Empty)),
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(Chunk "timeMillis" Empty, BsonInt32 90),
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(Chunk "counts" Empty,
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BsonDoc [(Chunk "input" Empty,BsonInt64 8),
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(Chunk "emit" Empty,BsonInt64 12),
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(Chunk "output" Empty,BsonInt64 4)]),
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(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
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> [[(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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> mapReduceResults c (fromString "test") res >>= allDocs
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[[(Chunk "_id" Empty,BsonString (Chunk "cat" Empty)),
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(Chunk "value" Empty,BsonDouble 6.0)],
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[(Chunk "_id" Empty,BsonString (Chunk "doc" Empty)),
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(Chunk "value" Empty,BsonDouble 1.0)],
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[(Chunk "_id" Empty,BsonString (Chunk "dog" Empty)),
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(Chunk "value" Empty,BsonDouble 3.0)],
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[(Chunk "_id" Empty,BsonString (Chunk "mouse" Empty)),
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(Chunk "value" Empty,BsonDouble 2.0)]]
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