cursor.toArray(callback) does not return an array of documents

Question

I want to return an array that contains documents of the decks collection. I can get the cursor to point to those documents and then I use the toArray() function to turn them into an array.

The problem is that I cannot return the converted array... please take a look at my code.

exports.find_by_category = function (category_id){
    var results = []; //Array where all my results will be
    console.log('Retrieving decks of category: ' + category_id);
    mongo.database.collection('decks', function(err, collection) {
        collection.find({'category_id': category_id}).toArray(function(err,items){
            results = items; //Items is an array of the documents
        });
    }); 
    return results; //The problems is here, results seems to be empty...
};

I honestly dont know what is going on since results is in the outer-scope. What am I doing wrong? How can I achieve returning results as an array of the found documents.

What's going on is that the call to .collection() is asynchronous. The results are not available immediately. — Pointy
– Pointy, Commented Dec 19, 2013 at 22:40

Community · Accepted Answer · 2017-05-23 12:26:07Z

15

As @Pointy pointed out, the line return results is executed synchronously, way before the call to collection.find has returned any results.

The way to solve this is to provide a callback to the function, like so:

exports.find_by_category = function (category_id, callback){ //Notice second param here  
    mongo.database.collection('decks', function(err, collection) {
       collection.find({'category_id': category_id}).toArray(function(err,items){
           if(err) callback(err);
           else callback(null, items);
        });
    }); 
};

For a better understanding of how callbacks work, check out this answer. And yes, async programming is hard at first, and does require some getting used to.

edited May 23, 2017 at 12:26

CommunityBot

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answered Dec 20, 2013 at 0:12

verybadalloc

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