Mongodb的mapreduce
时间：2021-07-01 10:21:17
帮助过：12人阅读
> db.person.find();
{ "_id" : ObjectId("593011c8a92497992cdfac10"), "name" : "xhj", "age" : 30, "address" : DBRef("address", ObjectId("59314b07e693aae7a5eb72ab")) }
{ "_id" : ObjectId("59301270a92497992cdfac11"), "name" : "zzj", "age" : 2 }
{ "_id" : ObjectId("593015fda92497992cdfac12"), "name" : "my second child", "age" : "i do not know" }
{ "_id" : ObjectId("592ffd872108e8e79ea902b0"), "name" : "zjf", "age" : 30, "address" : { "province" : "河南省", "city" : "南阳市", "building" : "桐柏县" } }
//使用聚合来做一个group by
> db.person.aggregate({$group : {_id: ‘$age‘, count : {$sum : 1}}})
{ "_id" : "i do not know", "count" : 1 }
{ "_id" : 2, "count" : 1 }
{ "_id" : 30, "count" : 2 }
//下面尝试用map reduce来做同样的group by效果
//很简单的逻辑 定义map函数 和reduce函数
> var m = function(){ emit(this.age,1) };
> var r = function(key,values){
... var sum = 0;
... values.forEach(function(val){
... sum += val;
... });
... return sum;
... }
//然后在person上执行mapreduce 这样会报错 需要一个optionsOrOutString
> db.person.mapReduce( m, r ).find();
assert failed : need to supply an optionsOrOutString
Error: assert failed : need to supply an optionsOrOutString
at Error (<anonymous>)
at doassert (src/mongo/shell/assert.js:11:14)
at assert (src/mongo/shell/assert.js:20:5)
at DBCollection.mapReduce (src/mongo/shell/collection.js:1343:5)
at (shell):1:11
2017-06-03T12:42:06.704+0800 E QUERY Error: assert failed : need to supply an optionsOrOutString
at Error (<anonymous>)
at doassert (src/mongo/shell/assert.js:11:14)
at assert (src/mongo/shell/assert.js:20:5)
at DBCollection.mapReduce (src/mongo/shell/collection.js:1343:5)
at (shell):1:11 at src/mongo/shell/assert.js:13
//加了一个而空的option 又说要有一个string或者object的out参数
> db.person.mapReduce( m, r,{} ).find();
2017-06-03T12:42:24.726+0800 E QUERY Error: map reduce failed:{
"errmsg" : "exception: ‘out‘ has to be a string or an object",
"code" : 13606,
"ok" : 0
}
at Error (<anonymous>)
at DBCollection.mapReduce (src/mongo/shell/collection.js:1353:15)
at (shell):1:11 at src/mongo/shell/collection.js:1353
//我尝试定义一个变量 不行
> var outstr;
> db.person.mapReduce( m, r,{out:outstr} ).find();
2017-06-03T12:42:45.502+0800 E QUERY Error: map reduce failed:{
"errmsg" : "exception: ‘out‘ has to be a string or an object",
"code" : 13606,
"ok" : 0
}
at Error (<anonymous>)
at DBCollection.mapReduce (src/mongo/shell/collection.js:1353:15)
at (shell):1:11 at src/mongo/shell/collection.js:1353
//后来我了解到out需要的一个collection 于是我加了一个字符串 ‘outt‘作为保存数据的集合名字
> db.person.mapReduce( m, r,{out:‘outt‘} ).find();
{ "_id" : 2, "value" : 1 }
{ "_id" : 30, "value" : 2 }
{ "_id" : "i do not know", "value" : 1 }
//此时outt中也保存了数据 我不明白的是 不定义out参数 不是应该可以直接find就可以了吗 为什么要多此一举呢
> db.outt.find();
{ "_id" : 2, "value" : 1 }
{ "_id" : 30, "value" : 2 }
{ "_id" : "i do not know", "value" : 1 }
因为遇到了这么多问题，所以看了Mongodb的文档（https://docs.mongodb.com/manual/reference/method/db.collection.mapReduce/），梳理了一下，总结如下：
命令方式：
- db.runCommand(
- {
- mapReduce: <collection>,
- map: <function>,
- reduce: <function>,
- finalize: <function>,
- out: <output>,
- query: <document>,
- sort: <document>,
- limit: <number>,
- scope: <document>,
- jsMode: <boolean>,
- verbose: <boolean>,
- bypassDocumentValidation: <boolean>,
- collation: <document>
- }
- )
简单方式：
- db.collection.mapReduce(map, reduce, {<out>, <query>, <sort>, <limit>, <finalize>, <scope>, <jsMode>, <verbose>})
Mongodb的mapreduce
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