Friday, 24 January 2014

Time Zone in Rails

There’s pretty good info out there about using time zones in Rails, and Rails itself does a lot of the heavy lifting. The Railscast pretty much covers it. It’s only missing a discussion of using Javascript to figure out the client browser’s time zone.

Time Zone from the Browser

To get the time zone from the browser, use the detect_timezone_rails gem. The instructions give you what you need to know to set up a form with an input field that will return the time zone that the browser figured out. That would work perfectly if you were implementing a traditional web site sign-up/sign-in form.
However, I needed to do something different. Since I’m using third party identity providers (Google, Twitter, Facebook, etc.) via the excellent Omniauth gems, I needed to be able to put the time zone as a parameter on the URL of the identity provider’s authorization request. Omniauth arranges for that parameter to come back from the identity provider, so it’s available to my app’s controller when I set up the session.
To add the parameter, I added this jQuery script to the head of the welcome page:
<script type="text/javascript">
          .each(function() {
            this.href = this.href + "?time_zone=" +
This added the time zone, appropriately escaped, to the URL for the identity provider (the href of the <a> elements). This worked because I had set each of the links to the identity providers to have class="time_zone", like this:
<div class="idp">
  <%= link_to image_tag("sign-in-with-twitter-link.png", alt: "Twitter"), 
    class: "time_zone" %></div>
In the controller, I did this (along with all the other logging in stuff):
if env["omniauth.params"] &&
  tz = Rack::Utils.unescape(env["omniauth.params"]["time_zone"])
  if user.time_zone.blank? 
    user.time_zone = tz!
    flash.notice = "Your time zone has been set to #{user.time_zone}." +
      " If this is wrong," +
      " please click #{view_context.link_to('here', edit_user_path(user))}" +
      " to change your profile."
  elsif user.time_zone != tz
    flash.notice = "It appears you are now in the #{tz} time zone. " +
      "Please click #{view_context.link_to(edit_user_path(user), 'here')}" +
      " if you want to change your time zone."
  logger.error("#{} (id: #{}) logged in with no time zone from browser.")
Of course, you may want to do something different in your controller.

Testing Time Zones

However you get your time zones, you need to be testing your app to see how it works with different time zones. YAML, at least for a Rails fixture, interprets something that looks like a date or time as UTC. So by default, that’s what you’re testing with. But that might not be the best thing.
I had read that a good trick for testing is to pick a time zone that isn’t the one your computer is in. Finding such a time zone might be hard if you have contributors around the world. I like the Samoa time zone for testing: Far away from UTC, not too many people living in the time zone, and it has DST.
If you want a particular time zone in your fixtures, you have to use ERB. For example, in my fixtures I might put this:
created_at: <%= Time.find_zone('Samoa').parse('2014-01-30T12:59:43.1') %>
And in the test files, something like this:
test "routines layout" do = 'Samoa'
  correct_hash = {
    routines(:routine_index_one)=> {, 01, 30)=> [


I found a few gotchas that I hadn’t seen mentioned elsewhere:
  • Rails applies the time zone magic when it queries the database, so if you change your time zone after you retrieve the data, then you have to force a requery, or the cached times will still be in the model. Shouldn’t be a problem when running tests, but is when using the console to figure things out
  • You can’t use database functions to turn times into dates, as these won’t use the time zone. No group by to_date(...) or anything like that

Tuesday, 7 January 2014

Self-Referential, Polymorphic, STI, Decorated, Many-to-Many Relationship in Rails 4


I wanted to model connections à la connections in LinkedIn or Facebook in a Rails application. This means a many-to-many association between instances of the same class. That caused me some grief trying to get it hooked up right because you can’t rely on Rails to figure everything out.
The other trick in my application is that the people involved in the connections might be users who have registered to use the application, or they might be people created by a registered user, but who aren’t registered to user the application.
Concretely, and hopefully more clearly, I have “users”, who have registered, and I have people who can be involved connections. In my app the people who aren’t registered users are “patients”.
In the course of trying to get this all to work I stumbled across three approaches to this type of problem:
  1. Polymorphic classes
  2. Single Table Inheritance (STI)
  3. Decorator pattern
The combination of the many-to-many combined with the two classes took a lot of work to get straight. The Rails Guides were a great starting point, but I find that specifying Rails associations can be tricky if it’s not completely straightforward, and especially when you start chaining them together.
In the end, I decided to go with the Decorator pattern. But I’ll start with the one I threw out first: Polymorphic.


I got pretty far with polymorphic associations, but I couldn’t figure out how I was going to get a list of all people (patients and users) connected to another person. I could either get all patients or all users from the methods that the Rails associations gave me, but not a list of all together.
I realized in writing the preamble above that I probably should have realized that what I was trying to model wasn’t really a polymorphic situation. Polymorphic in the examples I saw was used to connect an object to another object from any one of a number of unrelated classes. Of course, hindsight is 20/20.
This post convinced me that trying to get a list of all people wasn’t going to come naturally from a polymorphic approach, so I stopped pursuing it.

Single Table Inheritance

I got fired up about single table inheritance (STI) as I was reading about how to make the polymorphic approach work. A good brief write up is here: The Railscast is here: (sorry, it’s a pro Railscast so it’s behind a paywall).
Others say I shouldn’t do STI. People say it can cause problems. One problem is if the type of an object will change, and change because of user input, it’s hard to handle. The view and controller are fixed to a certain object, so you can’t change the object type based on user input.
So here’s the code. First, create the models:
rails g model person name:string type:string provider:string uid:string
rails g model link person_a:references person_b:references b_is:string
class Person < ActiveRecord::Base
  has_many :links, foreign_key: "person_a_id"
  has_many :people, :through => :links, :source => :person_b
  scope :patients, -> { where(type: "Patient") }
  scope :users, -> { where(type: "User") }
user.rb (obviously there will be functionality here, but this is what I needed to get the associations to work):
class User < Person
patient.rb (as with user.rb, functionality will come later):
class Patient < Person
class Link < ActiveRecord::Base
  belongs_to :person_a, class_name: "Person"
  belongs_to :person_b, class_name: "Person"
It was a little hard to get the associations to work. The key to making the has_many :links,... in person.rb work was the , class_name: "Person" on the association in link.rb.
With the above, I can do things like:
person = Person.find(1).first
That’s all pretty sweet, and I really considered using this approach. In fact, I may return to it. There’s a lot left to do with my application. However, I’m pretty sure that I will need to deal with cases like a registered user corresponding to multiple patients (e.g. people get created under different names). Eventually I need a way to consolidate them.


In the end, perhaps the simplest was the best. I just decorated a person with an instance of a user when the person is a registered user. (This allows multiple people for a user, which might be useful for consolidating duplicate people.)
Here’s what I did:
Generate the models:
rails g model link person_a:references person_b:references b_is:string
rails g model person user:references name:string
rails g model user uid:string name:string provider:string
require 'person_helper'

class Person < ActiveRecord::Base
  belongs_to :user
  has_many :links, foreign_key: :person_a_id
  has_many :people, through: :links, source: :person_b

  include PersonHelper
I thought the person model should have has_one instead of belongs_to, but that would put the foreign key in the wrong model.
require 'person_helper'

class User < ActiveRecord::Base
  has_many :identities, class_name: "Person"
  has_many :links, through: :identities
  has_many :people, through: :links, :source => :person_b

  include PersonHelper
module PersonHelper
  def users { |person| ! person.user_id.nil? }

  def patients { |person| person.user_id.nil? }
class Link < ActiveRecord::Base
  belongs_to :person_a, class_name: "Person"
  belongs_to :person_b, class_name: "Person"
With the above, I can do things like:
person = Person.find(1).first
user = User.find(2).first
Again, sweet. Same number of files at the STI version. Instead of subclassing, common functionality is handled by a mixin module.


Another thing people don’t seem to like about STI is that it’s easy to end up with a big table full of all sorts of columns used only in a few places. Most modern database management systems aren’t going to waste a significant amount of space for unused columns, so I’m not sure what the problem is.
However, it got me thinking if there isn’t a way in Rails to have more than one table under a model. Or more to the point, could you have a table for the base model class, and a different table for each of the subclasses, and have Rails manage all the saving a retrieving.
I’m sure I’m not the first person to think of this. But I’m not going to go looking for it right now.

Other Resources

Rails 4 guides on associations: and migrations:
Ryan Bates’ Railscast on self-referential associations:, and on polymorphic associations:

Thursday, 2 January 2014

Moving to rbenv and Installing Rails on LInux Mint 13

I'm back to doing a bit of Rails. As always, the world has moved on. Rails is at 4.0.2, and Ruby 2.0 is out. The Rails folks are recommending rbenv to manage different Ruby versions and their gems. I knew I still had some learning to do to be using rvm properly, so I decided to invest the learning time in learning rbenv, since that's what the mainstream was using.

First, I had to remove the lines at the end of my ~/.bashrc, ~/.profile, and ~/.bash_profile, and restart all my terminal windows.

I followed the rbenv installation instructions here:, including the optional ruby-build installation.

Then, I did:

rbenv install -l

that shows 2.0.0-p353 as the newest production version of MRI. So I did:

rbenv install 2.0.0-p353
rbenv rehash # Either this or the next was necessary to avoid trying to install Rails in the system gem directories.
rbenv global 2.0.0-p353
gem install rails
rbenv rehash # Don't forget this again

Now I was ready to test a new application:

rails new example
cd example
rails server

Then I pointed a browser to: http://localhost:3000, and voilà.

I'm not sure I want to leave the rbenv global in place...