"Cannot read property 'username' of undefined" in a Meteor Route? - meteor

Question

The page still renders as it should, but this appears in my console log.
Exception in callback of async function: TypeError: Cannot read
property 'username' of undefined
at null. (http://localhost:3000/router.js?39194109cc7760190c519a386d66c807ba19230c:48:9)
at boundNext (http://localhost:3000/packages/iron_middleware-stack.js?0e0f6983a838a6516556b08e62894f89720e2c44:424:31)
at Meteor.bindEnvironment (http://localhost:3000/packages/meteor.js?43b7958c1598803e94014f27f5f622b0bddc0aaf:983:22)
at onRerun (http://localhost:3000/packages/iron_router.js?a427868585af16bb88b7c9996b2449aebb8dbf51:520:9)
at boundNext (http://localhost:3000/packages/iron_middleware-stack.js?0e0f6983a838a6516556b08e62894f89720e2c44:424:31)
at Meteor.bindEnvironment (http://localhost:3000/packages/meteor.js?43b7958c1598803e94014f27f5f622b0bddc0aaf:983:22)
at onRun (http://localhost:3000/packages/iron_router.js?a427868585af16bb88b7c9996b2449aebb8dbf51:505:11)
at boundNext (http://localhost:3000/packages/iron_middleware-stack.js?0e0f6983a838a6516556b08e62894f89720e2c44:424:31)
at Meteor.bindEnvironment (http://localhost:3000/packages/meteor.js?43b7958c1598803e94014f27f5f622b0bddc0aaf:983:22)
at dispatch (http://localhost:3000/packages/iron_middleware-stack.js?0e0f6983a838a6516556b08e62894f89720e2c44:448:3)
Which points to the line indicated below:
var OnBeforeActions = {
loginRequired: function() {
if (!Meteor.userId()) {
this.render('hello');
} else {
this.next();
}
}
};
Router.onBeforeAction(OnBeforeActions.loginRequired, {
only: ['home']
});
Router.route('/', function() {
// this.render('homepage');
Router.go('/users/' + Meteor.userId());
});
Router.route('/users/:user_id', function() {
this.render('home', {
data: {
username: Meteor.users.findOne({
_id: this.params.user_id
}).username, // THIS LINE HERE
}
});
});
The idea is to redirect the user to his page if he is already logged in - and it appears to work as it should, but why am I getting this?

Answer 1

When the browser loads initially there will not be any data available (all the collections will be empty - until they are downloaded. Thus the result of the query Meteor.users.findOne({_id: this.params.user_id}) will be undefined.
This is the reason you can't read username out of undefined.
Since the query will re-run/it is reactive you just need to take account when it is null and it will sort itself out. user.username is only read if user is not null:
Router.route('/users/:user_id', function() {
var user = Meteor.users.findOne({_id :this.params.user_id});
this.render('home', {
data: {
username: user && user.username
}
});
});