Meteor observeChanges vs manual update - meteor

Question

I have a simple todo schema: (just a sample to draw my question)
{
title: {
type: string
},
value: {
type: string
},
author: {
type: object
},
"author._id": {
type: string
},
"author.firstName": {
type: string
},
"author.lastName": {
type: string
},
}
The author entries are from meteor.user. If the meteor user changes the firstName or lastName i have to update the todo. I have two possibilities:
observerChanges (server side) to users collection and update all todos from this user with the new firstname/lastname
if i call the user update method i can call a method to update all todos
when it's better to use cursor.observeChanges and when it's better to call a update method manual? And why?

Answer 1

As the comment says, you should not store the author name / email in the document if it is mutable:
Store the ID of the user only in the document, the UserID is immutable.
When building your ToDo template, look up the User information by ID: you would need to publish a Publication for user by Id, and subscribe to it on the client with the userId as parameter.
Meteor.publish('userById', function(userId) {
return Meteor.users.find({_id: userId}, {limit:1});
});
in your route / template.onCreated depending on your Router, assuming the document is called doc
this.subscribe('userById', this.doc.author._id);
in the template helper
Template.todoTemplate.helpers({
'Author': function() {
return Meteor.users.findOne({_id: this.doc.author._id});
}
});
and call the Author info in the template
<Template name="todoTemplate">
First Name: {{Author.first_name}}
Last Name: {{Author.last_name}}
</Template>

Answer 2

I think you shouldn't rely on the second method, because sometimes you (or your teammate) might forget to update it. Moreover, if you're denormalizing user data in other collections, users knowing Meteor might just call your Meteor.method or manipulate db from the browser console...
You can use this package:
meteor add matb33:collection-hooks
It adds some hooks to your mongo insert/update/remove call
For example:
Meteor.users.after.update(function (userId, doc, fieldNames, modifier, options) {
if (this.previous.firstName === doc.firstName && this.previous.lastName === doc.lastName) {
return;
}
Todos.update({'author._id': doc._id}, {
$set: {
'author.firstName': doc.firstName,
'author.lastName': doc.lastName,
}
})
}, {fetchPrevious: true})
(To update the Todos collection efficiently, make sure to add index to author field)
This is just a handier way than writing your own observeChanges, and better than manually updating Todos collection every time you update the users collection, because you might forgot to call it in some case, or some hacker user just calls Meteor.users.update(Meteor.userId(), {...}) perhaps...
But still, I think you should always add some auto-correct mechanism to avoid wrong data being displayed, because no matter which method you choose, some error will occur (maybe the server watching the db just crashes right after users update). You can check on the client side when displaying content, if author.firstName doesn't match Meteor.users.findOne(author._id) (but you have to publish the user though...), than call a method to tell the server to update it.