>> db.test.insert(

{x : 0}

)
>> var m = function()

{ emit(this.mod, this.x); }

>> var r = function(k,v) { total = 0; for(i in v)

{ total+= v[i]; }

return total; }
>> db.test.mapReduce(m, r,

{out : "test.out"}

)
{
"result" : "test.out",
"timeMillis" : 1,
"counts" :

{ "input" : 1, "emit" : 1, "output" : 1 }

,
"ok" : 1,
}

In the result, _id is undefined (it looks like null in the shell, but if you do a mongodump you can see it's type 6).

----------

This causes problems with replication (i.e., stops it completely).