MongoDB's MapReduce

A simple look at MapReduce, I try not to look at the detailed API to do a group effect, the results encountered a lot of problems, listed here, if others have encountered a similar bug, you can retrieve the results.

2. First look at the data of the person table
4. > Db.person. Find ();
6. {"_id": ObjectId ("593011c8a92497992cdfac10"), "name": "xhj", "Age": 30 , "address": Dbref ("address", ObjectId ("59314b07e693aae7a5eb72ab"))}
8. {"_id": ObjectId ("59301270a92497992cdfac11"), "name": "zzj", "Age ": 2}
10. {"_id ": ObjectId (" 593015fda92497992cdfac12 ")," name ":" Span style= "color:darkred" >my second child , "age ": "i do not know "}
12. {"_id": ObjectId ("592ffd872108e8e79ea902b0"), "name": "zjf", "Age": 30 , "address": {"Province": " Henan Province ", "City": " Nanyang ", "building" : " Tongbai County "}}
14. Use aggregations to make a group by
16. > db.person.aggregate ({$group: {_id: ' $age ', Count: {$sum: 1}}})
18. {"_id": "Ido not know", "Count": 1}
20. {"_id": 2, "Count": 1}
22. {"_id": +, "Count": 2}
24. Here's an attempt to use map reduce to do the same group by effect
26. Very simple logic to define the map function and the reduce function
30. > var m = function() {Emit (this. age,1)};
32. > var r = function(key,values) {
34. ... var sum = 0;
36. ... Values.foreach (function(val) {
38. ... sum + = val;
40. ... });
42. ... return sum;
44. ... }
48. And then execute the MapReduce on the person. This will cause an error to require a optionsoroutstring
50. > Db.person.mapReduce (M, R). Find ();
52. Assert Failed:need to supply an optionsoroutstring
54. Error:assert Failed:need to supply an optionsoroutstring
56. At Error (<anonymous>)
58. At Doassert (src/mongo/shell/assert.js:11:14)
60. At assert (Src/mongo/shell/assert.js:20:5)
62. At Dbcollection.mapreduce (src/mongo/shell/collection.js:1343:5)
64. at (Shell): 1:11
66. 2017-06-03t12:42:06.704+0800 E QUERY Error:assert failed:need to supply an optionsoroutstring
68. At Error (<anonymous>)
70. At Doassert (src/mongo/shell/assert.js:11:14)
72. At assert (Src/mongo/shell/assert.js:20:5)
74. At Dbcollection.mapreduce (src/mongo/shell/collection.js:1343:5)
76. at (Shell): 1:11 at Src/mongo/shell/assert.js:13
78. Added an empty option to have a string or an out parameter of object
80. > Db.person.mapReduce (M, r,{}). Find ();
82. 2017-06-03t12:42:24.726+0800 E QUERY error:map reduce failed:{
84. "errmsg": "Exception: ' Out ' have to is a string or an object",
86. "code": 13606,
88. "OK": 0
90. }
92. At Error (<anonymous>)
94. At Dbcollection.mapreduce (src/mongo/shell/collection.js:1353:15)
96. at (Shell): 1:11 at src/mongo/shell/collection.js:1353
98. I'm trying to define a variable.
100. > var outstr;
102. > Db.person.mapReduce (M, r,{out:outstr}). Find ();
104. 2017-06-03t12:42:45.502+0800 E QUERY error:map reduce failed:{
106. "errmsg": "Exception: ' Out ' have to is a string or an object",
108. "code": 13606,
110. "OK": 0
112. }
114. At Error (<anonymous>)
116. At Dbcollection.mapreduce (src/mongo/shell/collection.js:1353:15)
118. at (Shell): 1:11 at src/mongo/shell/collection.js:1353
120. Later I learned that a collection was needed so I added a string ' Outt ' as the collection name for the saved data
124. > Db.person.mapReduce (M, r,{out: ' Outt '}). Find ();
126. {"_id": 2, "value": 1}
128. {"_id": +, "value": 2}
130. {"_id": "Ido not know", "value": 1}
132. At this point Outt also saved the data I do not understand is not to define the out parameter should not be able to directly find it? Why superfluous?
134. > Db.outt. Find ();
136. {"_id": 2, "value": 1}
138. {"_id": +, "value": 2}
140. {"_id": "Ido not know", "value": 1}

Because of encountering so many problems, so looked at the MongoDB document (https://docs.mongodb.com/manual/reference/method/db.collection.mapReduce/), combed a bit, Summarized as follows:

Command mode:

2. Db.runcommand (
4. {
6. MapReduce: <collection>
8. Map: <function;
10. Reduce: <function;
12. Finalize: <function;
14. Out: <output>
16. Query: <document;
18. Sort: <document;
20. Limit: <number>
22. Scope: <document;
24. Jsmode: <boolean;,
26. Verbose: <boolean;,
28. Bypassdocumentvalidation: <boolean;,
30. Collation: <document>
32. }
34. )

Simple way:

1. Db.collection.mapReduce (map, reduce, {<out>, <query>, <sort>, <limit>, <finalize>, < Scope>, <jsmode>, <verbose>})

MongoDB's MapReduce