How to detect the re-reduce stage in MongoDB map/reduce? - mongodb

Question

I use the following map/reduce setup to collect some data into array:
map: function() { emit(this.key, [this.item]); },
reduce: function(key, values) {
var items = [];
values.forEach( function(value) {items.concat(value.item);} );
return items;
},
out: {reduce: "result_collection"}
I want to improve the code and detect if the resulting collection has been changed during the re-reduce stage (when mongo invokes reduce with the current content of the "result_collection").
In other words, how to know that any documents have been emitted by the Map contain "item" that does not exist in the "result_collection" yet (under the same key, of course)?
This information can help at some further processing stages e.g. query "result_collection" to get the documents that have been updated during the map/reduce stage.

Answer 1

If you must do this, use a finalize function to adjust the value after all reduction is finished. You'll have to add more logic to the reduce function to handle the modified output.
I'll show you an example with the simple map-reduce defined by the following map and reduce functions:
var map = function() { emit(this.k, this.v) }
var reduce = function(key, values) { return Array.sum(values) }
On documents that look like { "k" : 0, "v" : 1 }, the map-reduce defined by the above functions produces result documents that look like { "_id" : 0, "value" : 17 }. Define a finalize function to modify the final document:
var finalize = function (key, reducedValue) { return { "m" : true, "v" : reducedValue } }
Now modify reduce to handle an element of values that might be an object of the above form:
var reduce2 = function(key, values) {
var sum = 0;
for (var i = 0; i < values.length; i++) {
if (typeof values[i] == "object") { sum += values[i].v }
else { sum += values[i] }
}
return sum
}
Output looks like
{ "_id" : 0, "value" : { "m" : true, "v" : 14 } }
{ "_id" : 1, "value" : { "m" : true, "v" : 34 } }
{ "_id" : 2, "value" : { "m" : true, "v" : 8 } }
so you can tell what's been modified by value.m. Your further processing can set v.m to false so you'll see what hasn't been processed yet after each map-reduce.