用 `!` 容易出错，像下面这样就好理解多了：

```javascript
//get author's relationship
if (req.session.user && req.session.user._id) {
  Relation.getRelation(req.session.user._id, topic.author_id, ep.done('get_relation'));
} else {
  ep.emit('get_relation', null);
}
```