How to sum an array with meteor/mongo

Question

I've 18 documents in my collection movie. For each movie for example:

{
    title: "Test Movie 2",
    date: [20130808, 20130606],
    score: [ {"pete": 1, "mary": 1, "simon": 1, "pat": 2, "mike": 0},
             {"pete": 5, "mary": 5, "simon": 5, "pat": 0, "mike": 5}]
}

Now, I want to show the date and sum of the second document in the array 'score' on the client, like:

<div class="details">
    Test Movie 2:   20 points   20130606
</div>

Have somebody an idea how I can do that?

may be this http://stackoverflow.com/questions/12162681/mongodb-2-1-aggregate-framework-sum-of-array-elements-matching-a-name will help you — Freak, Aug 19 '13 at 12:03

Tarang · Answer 1 · 2013-08-19T14:05:23.187

1

You could use a transform, it might be better to define each document with a name and explicitly defining the points as name/value pair instead of points being the value for each persons name.

But this should work:

Movies.find({}, {transform: function(doc) {
    var total_points = 0;
    var people = doc.score[1]; //Second in array   
    for(point in people) {
        total_points += people[point]
    }

    doc.points = total_points;
    return doc;
}});

This should give you:

{
  title: "Test Movie 2",
  points: 20,
  date: [20130808, 20130606],
  score: [ {"pete": 1, "mary": 1, "simon": 1, "pat": 2, "mike": 0},
           {"pete": 5, "mary": 5, "simon": 5, "pat": 0, "mike": 5}]
}

edited Aug 19 '13 at 14:05

answered Aug 19 '13 at 12:27

Tarang

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I've decided for this approach, but it does not work. I've copied the code into my JavaScript file, but unfortunately it doesn't change my documents. I'll keep trying... – Sebastian Aug 19 '13 at 12:56
Oh sorry I forgot to `return doc` in the transform. Try with the updated code above – Tarang Aug 19 '13 at 14:05

Stephan Tual · Answer 2 · 2013-08-19T12:59:46.133

Mongo can likely do this outright, but you're not going to be able to do this directly by querying a collection due to limitation of the Mongo livedata package as of 0.6.5. Aggregation framework is also off-limits, and they seem to have pulled the 'hidden' method that allowed direct access to Mongo.

So your options are:

Use transform as per Akshat's answer - the best of both worlds
Aggregate manually in the client in a template helper. I recommend using _.js which comes 'free' with Meteor (again this might change, but you could always pull the library in manually later).

var sum = _.reduce(score[1], function(memo, num){ return memo + num; }, 0);

(I didn't test the above, but it should send you on the right track).
Aggregate upstream, during the insert/update/deletes, likely by observing changes on the collection and 'feeding in' the sum() of the elements you are inserting in either the same collection or an aggregate one.

Which method you use depends on where performance matters most to you, usually doing aggregates before you insert tend to avoid issues later on, and lightens load.

Good luck and let us know how you get on.

How to sum an array with meteor/mongo

2 Answers2