this.body does not be assigned when mongo returns data

[guoshencheng]

I am a fresh man to node.js koa and mongo,When I use then code like demo

app.use(router.get('/', function *(next) {
    this.mongo.db('test').collection('user').findOne({}, function (err, doc) {
        console.log(err, doc);
        this.body = doc;
    });
}));

I can see doc log in console but this.body is empty, I tried assign this.body before use mongodb like

app.use(router.get('/', function *(next) {
    this.body = {"type":"test"};
    this.mongo.db('test').collection('user').findOne({}, function (err, doc) {
        console.log(err, doc);
    });
}));

It returns data, what can I do to make server return data after mongo returns data

[nswbmw]

try:

app.use(router.get('/', function *(next) {
    this.body = yield this.mongo.db('test').collection('user').findOne();
}));

[guoshencheng]

@nswbmw thanks for you answer! My problem has been solved!