The `waitOn` prototype property can take a handle, an array of handles, or a function. Take a look here: https://github.com/EventedMind/meteor-iron-router/blob/dev/lib/client/route_controller.js#L84

So you can do this:

```javascript
PostsController = RouteController.extend({
  waitOn: function () {
    return Meteor.subscribe('posts', this.params._id);
  }
});
```