MongoDB Map/Reduce struggles - mapreduce

Question

Given a document like this:
{ "_id": {
"$oid": "4d88ca367d190a0aa4e27806" }, "Rows": [
{
"Columns": {
"Date": "Tue, 02 Aug 2011 00:00:00 GMT -04:00",
"Col2": 33
"Col3": 44
}
},
{
"Columns": {
"Date": "Mon, 17 Oct 2011 00:00:00 GMT -04:00",
"Col2": 55
"Col3": 66
}
} ], "DocName": "For My Eyes Only", "DocIdentifier": 3322445 }
and the following Map/Reduce functions:
function() {
this.Rows.forEach(function(bulkcol) {
emit(this.DocName, { TrendValue: bulkcol.Columns.Col2 });
});
};
function(key, values) {
var sum = 0;
values.forEach(function(currTrend) {
sum += currTrend.TrendValue;
});
return {DocName: key, TrendValue: sum};
};
I get the following output:
{
"_id": null,
"value": {
"DocName": null,
"TrendValue": 88
}
}
Why is the DocName null?

Answer 1

The problem is very much as Srdjan indicated - inside your forEach function, you don't actually have a reference to the parent document.
If you change your map function to this, it works:
m = function() {
var docName = this.DocName;
this.Rows.forEach(function(bulkcol) {
emit(docName, { TrendValue: bulkcol.Columns.Col2 });
});
};
So, assign the doc name into a variable before the loop, and then use that within it

Answer 2

If I'm looking at this right, and I'd like to think that I am, the this in the emit function is not what you're expecting it to be.
Since it's inside a function, that this refers to each Row, not the parent document. Try this:
function() {
this = parent;
this.Rows.forEach(function(bulkcol) {
emit(parent.DocName, { TrendValue: bulkcol.Columns.Col2 });
});
};