Sho Fukamachi Online

I have covered forcing ActiveRecord to respect UUID data types in Migrations before. That helps us create our database – now what about in use? We need to create the UUIDs and store them in the database.

These examples all rely on the uuidtools gem, so install that if you haven’t already (and require it somewhere in environment.rb).

1. Setting a UUID using ActiveRecord callbacks

If you don’t need the UUID in the object upon creation but only want to ensure it’s there upon save, do this. Suggestion initially from this page, changes are mine.

We will use the before_create callback to ask AR to add a UUID of our choosing before the record is saved.

Add this to your lib directory:

# lib/uuid_helper.rb
require 'uuidtools'
 
module UUIDHelper
  def before_create
    self.id = UUID.random_create.to_s
  end
end

And now include this in your models:

class Airframe < ActiveRecord::Base
  include UUIDHelper
 
  #my stuff
 
end

>> Airframe.new
=> #< Airframe id: nil, maker_id: nil>
>> Airframe.create!
=> #< Airframe id: "1a82a408-32e6-480e-941d-073a7e793299", maker_id: nil>

2. Initialising a model with a UUID

If you want the UUID in the model before save, i.e. upon initialisation, we have to get a little more fancy:

# lib/uuid_init.rb
require 'uuidtools'
 
module UUIDInit
  def initialize(attrs = {}, &block) 
   super 
   ['id'] = UUID.random_create.to_s
  end
end

Now include this in your models:

class Flightpath  < ActiveRecord::Base
 
  include UUIDInit
 
  # my stuff
 
end

>> Flightpath.new
=> #< Flightpath created_at: nil, id: "5e5bcd63-070d-4252-8556-2876ddd83b54">

Be aware that it will conflict with any other initialisation you do in there, so you might want to simply copy in the whole method if you need other fields upon initialisation:

class User < ActiveRecord::Base
 
  def initialize(attrs = {}, &block) 
   super 
   ['balance'] = 0.0
   ['id'] = UUID.random_create.to_s
  end
 
end

>> User.new
=> #

3. Sessions

All this is very well for your own models, but what about Rails’ inbuilt sessions? By default, they want an autoincrementing integer primary key.

The good news is it’s easy to override. Your migration should look like this:

create_table "sessions", :id => false, :force => true do |t|
  t.string   "session_id"
  t.text     "data"
  t.datetime "updated_at"
  t.datetime "created_at"
end

Now add this to your environment.rb file:

# config/environment.rb
CGI::Session::ActiveRecordStore::Session.primary_key = 'session_id'

And this to your Application Controller:

# app/controllers/application.rb
class ApplicationController < ActionController::Base
 
before_filter :config_session # at the top, if possible
 
def config_session
  session.model.id = session.session_id
end
 
end

And voila, your session store is using the session_id as its primary key. I don’t see any point in using a UUID for your sessions’ PK, but if you want to you’ll find an example override class in:

actionpack/lib/action_controller/session/active_record_store.rb.

Remember to drop any preexisting sessions table in your database, or it will likely complain of null ids when you switch to session_id as your primary key.