How to keep track of model history with mapping table in Ruby on Rails?

You're looking for the acts_as_audited plugin. It provides an audits table and model to be used in place of your map.

To set it up run the migration and add the following to your user address model.

class UserAddress < ActiveRecord::Base
  belongs_to :user
  acts_as_audited
end

Once you've set it up, all you need to do is define an address method on order. Something like this:

class Order < ActiveRecord::Base
   belongs_to :user
   attr_reader :address

   def address
     @address ||= user.user_address.revision_at(updated_at)
   end
end

And you can access the users' address at the time of order completion with @order.address

revision_at is a method added to an audited model by acts_as_audited. It takes a timestamp and reconstructs the model as it was in that point of time. I believe it pieces the revision together from the audits up on that specific model before the given time. So it doesn't matter if updated_at on the order matches a time exactly.

Thought I'd add an updated answer. Seems the paper_trail gem has become the most popular one for versioning in Rails. It supports Rails 4 as well.

https://github.com/airblade/paper_trail

From their readme:

To setup and install:

gem 'paper_trail', '~> 3.0.6'
bundle exec rails generate paper_trail:install
bundle exec rake db:migrate

Basic Usage:

class Widget < ActiveRecord::Base
  has_paper_trail
end

For a specific instance of the Widget class:

v = widget.versions.last
v.event                     # 'update' (or 'create' or 'destroy')
v.whodunnit                 # '153'  (if the update was via a controller and
                               #         the controller has a current_user method,
                               #         here returning the id of the current user)
v.created_at                # when the update occurred
widget = v.reify            # the widget as it was before the update;
                               # would be nil for a create event

I've only played with it but I'm about to start a pretty ambitious site which will require good versioning of certain classes and I've decided to use paper_trail.

===EDIT====

I have implemented the paper_trail gem in production at www.muusical.com and it has worked well using the above. The only change is that I am using gem 'paper_trail', '~> 4.0.0.rc' in my Gemfile.

From a data architecture point of view, I suggest that to solve your stated problem of

...when an order is placed, it will always be able to reference the user address that was used at the time of order placement.

... you simply copy the person's address into an Order model. The items would be in OrderItem model. I would reformulate the issue as "An order happens at a point in time. The OrderHeader includes all of the relevant data at that point in time."

Is it non-normal?

No, because the OrderHeader represents a point in time, not ongoing "truth".

The above is a standard way of handling order header data and removes a lot of complexity from your schema as opposed to tracking all changes in a model.

--Stick with a solution that solves the real problem, not possible problems--does anyone need a history of the user's changes? Or do you just need the order headers to reflect the reality of the order itself?

Added: And note that you need to know which address was eventually used to ship the order/invoice to. You do not want to look at an old order and see the user's current address, you want to see the address that the order used when the order was shipped. See my comment below for more on this.

Remember that, ultimately, the purpose of the system is to model the real world. In the real world, once the order is printed out and sent with the ordered goods, the order's ship-to isn't changing any further. If you're sending soft goods or services then you need to extrapolate from the easier example.

Order systems are an excellent case where it is very important to understand the business needs and realities--don't just talk with the business managers, also talk with the front-line sales people, order clerks, accounts receivable clerks, shipping dept folks, etc.

Use the Vestal versions plugin for this:

Refer to this screen cast for more details.

class Address < ActiveRecord::Base
  belongs_to :user
  versioned
end


class Order < ActiveRecord::Base
   belongs_to :user

   def address
     @address ||= (user.address.revert_to(updated_at) and user.address)
   end
end