return variable from a nested callback in node.js

I have the results of my query in result but I can not pass it to browser, it is showing empty. how can I return the result to res.send('result')?

myApp.get('/agg',function(req,res){
MongoClient.connect(url, function(err, db) {
    // Execute aggregate, notice the pipeline is expressed as an Array
    db.collection('projects').aggregate([{$match:{school_state:"DC"}}, {$group:{_id: "$primary_focus_subject",total_amount:{$sum: "$total_donations"}}}],function(err,result){
    console.log(result);

    db.close();
    });
});

res.send(result);

});

via solarisan