CODEWITHSUNDEEP
javascriptjsonnode.js
Alpaus
Alpaus

Reputation: 646

Node.js/Javascript searching in arrays of JSON objects

I am fairly new to Javascript and JSON objects and taking my first foray into Node.Js.

I have a problem for which my solution is working perfectly but it seems very much like a hack and very inefficient. I'm wondering if there's a better way to do this.

I have two arrays of JSON objects coming from a MongoDB.

  1. Activities Array

    [{"_id":"538599d44ec097f59aa9f49d", "type":"A", "value":0.5},
 {"_id":"538599d44ec097f59aa9f49e", "type":"A", "value":0.25},
 {"_id":"538599d44ec097f59aa9f49f", "type":"B", "value":1.0},
 {"_id":"538599d44ec097f59aa9f4a0", "type":"B", "value":0.5}]

  2. Transaction Counts Array

    [{"_id":"538599d44ec097f59aa9f4a0", "total":3},
 {"_id":"538599d44ec097f59aa9f49e", "total":6,
 {"_id":"538599d44ec097f59aa9f49d", "total":2,]

Now basically I want to get all of the info for EVERY item out of the activities array and lookup the matching total from the transaction counts array. The transaction counts array won't have an entry for that _id if the total is 0.

Desired results:

    [{"_id":"538599d44ec097f59aa9f49d", "type":"A", "value":0.5, "total":2},
     {"_id":"538599d44ec097f59aa9f49e", "type":"A", "value":0.25, "total":6},
     {"_id":"538599d44ec097f59aa9f49f", "type":"B", "value":1.0, "total":0},
     {"_id":"538599d44ec097f59aa9f4a0", "type":"B", "value":0.5, "total":3}]

I would do this in 5 seconds in SQL using a left join from activities to transactions but can't figure out how to emulate this efficiently in MongoDb so doing it in the application instead. This is how I am currently doing it:

    var results = [];
    for (i=0; i<data.activities.length;i++) {
        var item_count = 0;
        for (j=0; j<data.txns.length;j++) {
            if (data.activities[i]._id === data.txns[j]._id) {
                item_count = data.txns[j].total;
            }
        }
        var new_json = data.activities[i];
        new_json.item_count = item_count;
        results.push(new_json);
    }

So this successfully gives me the results variable containing an array of JSON objects, each now containing the activity details and that activity's corresponding transaction total.

The cheaper way to do this seems to be to turn the nesting inside out, iterate over the transactions array first, and for each transaction object find the corresponding activity details. However, it still seems 'wrong' to be nesting for loops like this for something that seems like it should be a simple lookup from one set of data to another by id.

Thanks in advance.

Aaron

Upvotes: 0

Views: 597

Answers (2)

Matt Stevens
Matt Stevens

Reputation: 1124

Purely from an algorithm efficiency pov, you could benefit, first by not completing the inner loop when you have found the correct _id key, second by not repeatedly iterating over objects you know have already been matched. This will reduce the algorithm's run time by a factor of 4.

For example (marked with comments as my js is rusty, this is pseudocode);

 var results = [];
    for (i=0; i<data.activities.length;i++) {
        var item_count = 0;
        for (j=0; j<data.txns.length;j++) {
            if (data.activities[i]._id === data.txns[j]._id) {
                item_count = data.txns[j].total;
                // data.txns.remove(i);
                // break;
            }
        }
        var new_json = data.activities[i];
        new_json.item_count = item_count;
        results.push(new_json);
    }

Upvotes: 0

elclanrs
elclanrs

Reputation: 94121

Here's a different, perhaps more readable way to do what you want:

var extend = function(x, y) {
  Object.keys(y).forEach(function(k){x[k] = y[k]});
  return x;
};

var result = activities.map(function(a) {
  return extend(a, transactions.filter(function(t) {
    return t._id == a._id;
  })[0]);
});

The transaction counts array won't have an entry for that _id if the total is 0.

By the looks of your desired output, I'm not sure I understand what you mean there, but you could add a simple test inside the filter:

if (!t.total) return;

Upvotes: 1

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