如何在java中的mapReduce中调用mongodb服务器端函数

Question

i have stored two functions in the "system.js" collection named "mapfun" and "reducefun" and i am trying to call those function from java. i am trying to call these function through MapReduceCommand. i am not able to call these functions. can anyone please help me with this. the two function looks like this //mapfun

{ "_id" : "mapfun","value" : { "code" : "function(){var criteria; if(this.speed > 70 ){ criteria="overspeed";emit(criteria,this.speed); } }" } }

//reducefun

{ "_id" : "reducefun", "value" : { "code" : "function(key,speed){ var total=0; for(var i=0;i<speed.length;i++){ total=total+speed[i]; } return total/speed.length; }" } }

and my mapreducecommand looks like this

MapReduceCommand command=new MapReduceCommand(collection, map, reduce, null, MapReduceCommand.OutputType.INLINE, null);
MapReduceOutput output=collection.mapReduce(command);

i have passed the map and reduce function as strings in which i have called the mapfun and reducefun respectively and it looks like this

String map = "function (){var x=mapfun();"
                + "return x;};";

String reduce = "function(key, speed) {var y=reducefun(key,speed);"
                + "return y;};";

what am i doing wrong here and how to correct this kindly help me with this.

Answer 1

The underlying problem is that the this variable is reassigned for each function call.

To demonstrate, I ran a short test from the mongo shell.

Fist create a collection containing 1 document:

MongoDB Enterprise replset:PRIMARY> db.maptest.find({},{_id:0})
{ "canary" : "tweet" }

Then create a stored function that will take an argument emit 2 values, one using this and one using the passed argument:

MongoDB Enterprise replset:PRIMARY> db.system.js.find()
{ "_id" : "testfun", "value" : { "code" : "function(arg){emit(\"this.canary\",this.canary),emit(\"arg.canary\",arg.canary)}" } }

Then use the stored function in a mapReduce call:

MongoDB Enterprise replset:PRIMARY> db.maptest.mapReduce(function(){testfun(this)},function(a,b){return {a:a,b:b}},{out:{inline:1}})
{
    "results" : [
        {
            "_id" : "arg.canary",
            "value" : "tweet"
        },
        {
            "_id" : "this.canary",
            "value" : undefined
        }
    ],
...

As you can see, this.canary was undefined, but arg.canary contained the value from the input document.

The mapReduce framework assigns this to the document currently being examined when calling the map function. When the stored function is called from within the map function it gets its very own this . However, by calling the function as testfun(this) the original this context of the map function is provided to the stored function as an argument.