can not use underscore.js in reduce function when do mapreduce on mongodb using mongoose

Question

I have a movie collection, each movie has several stars, for example:

{
    movie: "Pride & Prejudice",
    starList: ["Keira Knightley", "Matthew Macfadyen"]
}


{
    movie: "Begin Again",
    starList: ["Keira Knightley", "Mark Ruffalo"]
}

And I want to get an Inverted Index, each star has several movies

{ star: "Keira Knightley", movieList: ["Pride & Prejudice", "Begin Again"] }

This can be accomplished by using mapreduce on mongodb. I use nodejs with mongoose driver. Following is the code:

var _ = require("underscore");

var o = {};

o.scope = { _: _ };

o.map = function()
{
    var movie = this.movie;
    this.starList.forEach(function(star)
    {
        emit(star, { movieList: [movie]});
    });
};

o.reduce = function(k, vals)
{
    var movieList = _.flatten(_.pluck(vals, "movieList"));
    return { movieList: movieList};
};

Movie.mapReduce(o).then(function(results)
{
    console.log(JSON.stringify(results, null, 4));
});

I got an error which shows I cannot use underscore.js in reduce function.

MongoError: TypeError: _.pluck is not a function :
_funcs2@:8:34

    at Function.MongoError.create (/app/node_modules/mongoose/node_modules/mongodb/node_modules/mongodb-core/lib/error.js:31:11)
    at commandCallback (/app/node_modules/mongoose/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:1187:66)
    at Callbacks.emit (/app/node_modules/mongoose/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:119:3)
    at null.messageHandler (/app/node_modules/mongoose/node_modules/mongodb/node_modules/mongodb-core/lib/topologies/server.js:358:23)
    at Socket.<anonymous> (/app/node_modules/mongoose/node_modules/mongodb/node_modules/mongodb-core/lib/connection/connection.js:292:22)
    at emitOne (events.js:77:13)
    at Socket.emit (events.js:169:7)
    at readableAddChunk (_stream_readable.js:153:18)
    at Socket.Readable.push (_stream_readable.js:111:10)
    at TCP.onread (net.js:536:20)

chridam · Accepted Answer · 2016-08-23 16:54:30Z

1

A very simple approach which uses the aggregation framework follows:

Movie.aggregate([
    { "$unwind": "$starsList" },
    {
        "$group": {
            "_id": "$starsList",
            "movieList": {"$push": "$movie"}
        }
    },
    {
        "$project": {
            "_id": 0,
            "star": "$_id",
            "movieList": 1
        }
    }
]).exec(callback);

or using the fluent aggregate() builder API:

Movie.aggregate()
     .unwind("starsList")
     .group({
        "_id": "$starsList",
        "movieList": {"$push": "$movie"}
     })
     .project({
        "_id": 0,
        "star": "$_id",
        "movieList": 1
     })
     .exec(callback);

answered Aug 23, 2016 at 16:54

chridam

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2 Comments

KiwenLau Over a year ago

In addition, the "startsList" should be "starList"

KiwenLau Over a year ago

Thanks, it works, but I cannot accept your answer because the problem is about "using underscore in mapreduce"

Mykola Borysyuk · Accepted Answer · 2016-08-23 16:47:17Z

0

I may be wrong here. But i think when you try to acess underscore methods in reduce method your _ variable doesn't have all methods or for access you must use different approach. I would suggest to try like this

_['pluck'](....)

And also check _ variable to see if you got all methods in scope correctly.

Hope this helps.

answered Aug 23, 2016 at 16:47

Mykola Borysyuk

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1 Comment

KiwenLau Over a year ago

How to check _ variable in reduce function? It seems I can not using console.log() in reduce function

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