Meteor Subscription timeline

Console output

at Subscribing to post#undefined; comments are:
at Subscribed; comments are:
at Startup; comments are:
at Subscription complete; comments are:

at Setting session var to 1; comments are:
at Set to 1; comments are:
at Subscribing to post#1; comments are:
at Subscribed; comments are:
at Flushed; comments are:
at Subscription complete; comments are: first post on #1 - second post on #1 

at Setting session var to 2; comments are: first post on #1 - second post on #1
at Set; comments are: first post on #1 - second post on #1 
at Subscribing to post#2; comments are: first post on #1 - second post on #1 
at Subscribed; comments are: first post on #1 - second post on #1 
at Flushed; comments are: first post on #1 - second post on #1 
at Subscription complete; comments are: first post on #1 - second post on #1 - first post on #2 - second post on #2

at Later; comments are: first post on #2 - second post on #2 

timeline-tester.js

Comments = new Meteor.Collection('comments');
// {post_id: '1', text: 'hi there..'}

if (Meteor.is_client) {
  function log_comments(where) {
    var cs = Comments.find({}, {reactive: false}).map(function(c) { return c.text; }).join(' - ') ;
    
    console.log("at " + where + "; comments are: " + cs);
    
  }
  
  Meteor.autosubscribe(function() {
    var post_id = Session.get('post_id');
    
    log_comments("Subscribing to post#" + post_id);
    Meteor.subscribe('comments', post_id, function() {
      log_comments('Subscription complete');
    });
    log_comments("Subscribed");
  })
  
  Meteor.startup(function() {
    log_comments('Startup');
    
    // wait long enough for everything to happen
    Meteor.setTimeout(function(){
      console.log('');
      log_comments('Setting session var to 1');
      Session.set('post_id', 1);
      log_comments('Set to 1');
      Meteor.flush()
      log_comments('Flushed');
    }, 5000);
    
    // and wait even longer
    Meteor.setTimeout(function() {
      console.log('');
      log_comments('Setting session var to 2');
      Session.set('post_id', 2);
      log_comments('Set');
      Meteor.flush()
      log_comments('Flushed');
    }, 10000)
    
    Meteor.setTimeout(function() {
      console.log('');
      log_comments('Later');
    }, 15000);
  });
}

if (Meteor.is_server) {
  Meteor.publish('comments', function(post_id) {
    return Comments.find({post_id: post_id});
  });
  
  // add some fixture data
  if (Comments.find().count() === 0) {
    Comments.insert({post_id: 1, text: 'first post on #1'});
    Comments.insert({post_id: 1, text: 'second post on #1'});
    Comments.insert({post_id: 2, text: 'first post on #2'});
    Comments.insert({post_id: 2, text: 'second post on #2'});
  }
}