group by equivalent?

[headfox23]

Hey ho,
is there a way to do a classical group by as known in normal transactional databases?

I mean e.g. i have multiple entries with a field category. Now i want to have a list of categories...

Of course i can do the deduplication after the query... but it would be nice if the DB could handle this....
Currently i do this:

function getCategoriesFromDB(callback)
{
    DBStuff.find({}).filter(function(x){ return x.category != null }).map(function(x){ return { category: x.category } }).exec(function(err,categories) { 
        if(err != null)
        {
            callback(err,null);
        }
        if(categories == null)
        {
            callback(err,"no categories found");
        }

        //deduplication...
        function hash(o){
            return o.category;
        }    

        var hashesFound = {};

        categories.forEach(function(o){
            hashesFound[hash(o)] = o;
        })

        var uniqueCategories = Object.keys(hashesFound).map(function(k){
            return hashesFound[k];
        })
        //deduplication finished
        callback(err,uniqueCategories);
    });
}

[Ivshti]

You can use .reduce() to do it, example:

.reduce(function(cur, obj) {
    cur[obj.category] = cur[obj.category] || [];
    cur[obj.category].push(obj);
}, { })