Why should I ensure a unique timestamp in Meteor? - meteor

Question

I'm learning meteor and reading the todos example:
$ curl https://install.meteor.com | sh
$ meteor create --example todos
In todos/server/bootstrap.js around line 50 there are lines as follows.
var timestamp = (new Date()).getTime();
for (var i = 0; i < data.length; i++) {
var list_id = Lists.insert({name: data[i].name});
for (var j = 0; j < data[i].contents.length; j++) {
var info = data[i].contents[j];
Todos.insert({list_id: list_id,
text: info[0],
timestamp: timestamp,
tags: info.slice(1)});
timestamp += 1; // ensure unique timestamp.
}
}
I wonder why do we need to ensure the timestamp is unique? Is it required for meteor/mongodb or just application-specific?

Answer 1

The timestamp isn't required or doesn't have to be unique, its application specific in this case.
The use case here is just so that each todo item has an sequential timestamp so that it displays it in the correct order as can be seen in the client js, oldest at the top and newest at the bottom.
Template.todos.todos = function () {
...
return Todos.find(sel, {sort: {timestamp: 1}});
};
The server js uses adds 1 microsecond to each item in the loop to simulate the sort order that would perhaps be put in with with an actual user. Its only done once to create an example database.
If there was no sort order/sequential timestamps the todos would be sorted by the records _id, which would make their order at best, random. The reason for this is meteor uses Meteor.uuid() to generate its _id fields which are random & without sequentiality.