Example of map/reduce with multiple keys to group by?

[JoshL]

Can someone post an example of how you would do a map reduce when you
want to group by multiple keys? The docs give a great group by
example for "select a,b,sum(c) csum from coll where active=1 group by
a,b" but none of the map reduce examples I can find take into account
a AND b, just a.

If anyone has a quick example of doing a map reduce with multiple keys
they can post, it would be appreciated.

[Eliot Horowitz]

you just emit an object as your key
emit( { a : 1 , b : 1 } , ... )

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[JoshL]

Tried that. No joy.

Can someone PLEASE just post a complete example like there is for
group by?

Much appreciated.

[Eliot Horowitz]

Why didn't that work...

[JoshL]

I dont know. I have this group by:

r = function(obj, prev) { prev.csum += parseFloat(obj.value); ccount+
+; }

db.vector.group({key: {site:true, goal:true, tags:true}, cond: {value:
{ $exists : true }}, reduce: r, initial: {csum: 0, ccount: 0}});

and this works fine but craps out at 10,000 unique rows.

I tried to convert this to map reduce based on what you said,

m = function() { emit{site:1, goal:1, tags:1} }

r = function(k,v) { var csum = 0; var ccount = 0; for(var i in v)
{ csum += parseFloat(v[i].value); ccount++ }; return {csum, ccount} }

and it didnt work. As I mentioned I dont have a complete example to
go off to do this so not sure how to proceed.

[Eliot Horowitz]

You should read the docs on emit and look at the basic examplex
this is wrong:

> m = function() { emit{site:1, goal:1, tags:1} }

try
> m = function() { emit{site:1, goal:1 } , { tags:1} }

[JoshL]

Eliot -

Ive READ the docs multiple times and looked at the basic examples.

This is why im asking someone to give me a BASIC EXAMPLE for a multi-
key group by using map reduce.

Can someone provide this?

On Apr 1, 4:12 pm, Eliot Horowitz <eliothorow...@gmail.com> wrote:
> You should read the docs on emit and look at the basic examplex
> this is wrong:
>
>
>
> > m = function() { emit{site:1, goal:1, tags:1} }
> try
> > m = function() { emit{site:1, goal:1 } , { tags:1} }

[Ludovic Levesque]

You can read this PDF: http://rickosborne.org/download/SQL-to-MongoDB.pdf

[JoshL]

Now that's helpful. Thank you Ludovic.