mongoDB map/reduce minus the reduce

Question

I have some 25k documents (4 GB in raw json) of data that I want to perform a few javascript operations on to make it more accessible to my end data consumer (R), and I would like to sort of "version control" these changes by adding a new collection for each change, but I cannot figure out how to map/reduce without the reduce. I want a one-to-one document mapping—I start out with 25,356 documents in collection_1, and I want to end up with 25,356 documents in collection_2.

I can hack it with this:

var reducer = function(key, value_array) {
    return {key: value_array[0]}
}

And then call it like:

db.flat_1.mapReduce(mapper, reducer, {keeptemp: true, out: 'flat_2'})

(My mapper only calls emit once, with a string as the first argument and the final document as the second. It's a collection of those second arguments that I really want.)

But that seems awkward and I don't know why it even works, since my emit call arguments in my mapper are not equivalent to the return argument of my reducer. Plus, I end up with a document like

{
    "_id": "0xWH4T3V3R", 
    "value": {
        "key": {
            "finally": ["here"],
            "thisIsWhatIWanted": ["Yes!"]
        }
    }
}

which seems unnecessary.

Also, a cursor that performs its own inserts isn't even a tenth as fast as mapReduce. I don't know MongoDB well enough to benchmark it, but I would guess it's about 50x slower. Is there a way to run through a cursor in parallel? I don't care if the documents in my collection_2 are in a different order than those in collection_1.

mongoDB map/reduce minus the reduce

5 Answers