mapreduce between consecutive documents - mongodb

Question

Setup:
I got a large collection with the following entries
Name - String
Begin - time stamp
End - time stamp
Problem:
I want to get the gaps between documents, Using the map-reduce paradigm.
Approach:
I'm trying to set a new collection of pairs mid, after that I can compute differences from it using $unwind and Pair[1].Begin - Pair[0].End
function map(){
emit(0, this)
}
function reduce(){
var i = 0;
var pairs = [];
while ( i < values.length -1){
pairs.push([values[i], values[i+1]]);
i = i + 1;
}
return {"pairs":pairs};
}
db.collection.mapReduce(map, reduce, sort:{begin:1}, out:{replace:"mid"})
This works with limited number of document because of the 16MB document cap. I'm not sure if I need to get the collection into memory and doing it there, How else can I approach this problem?

Answer 1

The mapReduce function of MongoDB has a different way of handling what you propose than the method you are using to solve it. The key factor here is "keeping" the "previous" document in order to make the comparison to the next.
The actual mechanism that supports this is the "scope" functionality, which allows a sort of "global" variable approach to use in the overall code. As you will see, what you are asking when that is considered takes no "reduction" at all as there is no "grouping", just emission of document "pair" data:
db.collection.mapReduce(
function() {
if ( last == null ) {
last = this;
} else {
emit(
{
"start_id": last._id,
"end_id": this._id
},
this.Begin - last.End
);
last = this;
}
},
function() {}, // no reduction required
{
"out": { "inline": 1 },
"scope": { "last": null }
}
)
Out with a collection as the output as required to your size.
But this way by using a "global" to keep the last document then the code is both simple and efficient.