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Freeze a belongs_to association in Active Record.
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= Association Freezer Freeze an Active Record belongs_to association to ignore any future changes. == Install First specify it in your Rails config. config.gem 'ryanb-association-freezer', :lib => 'association_freezer', :source => 'http://gems.github.com' And then install it. rake gems:install Follow the instructions below to finish setup. Rails 2.1 or later required. == Scenario Let's say we have an Order model with a couple belongs_to associations for setting the billing and shipping address. class Order < ActiveRecord::Base belongs_to :shipping_address, :class_name => 'Address' belongs_to :billing_address, :class_name => 'Address' # ... end Consider this scenario. A user comes to the site and places an order. Then he comes back a month later and changes his address. Now our Order model is incorrect, it will point to this new address instead of the old one he placed the order with. There's a couple ways to solve this, one is to create a new Address model whenever one edits the address. Another is to freeze the address attributes upon placing an order so it isn't effected when the original Address changes. This gem will help with the latter. == Usage First you need to generate a binary (or blob) column to hold each frozen assoiation. The name of the column must start with "frozen_" and end with the association name. You can do that with a migration like this: script/generate migration add_frozen_addresses_to_orders \ frozen_billing_address:binary frozen_shipping_address:binary Next, enable the association freezer on your model. This must be done AFTER the associations have been specified. class Order < ActiveRecord::Base belongs_to :shipping_address, :class_name => 'Address' belongs_to :billing_address, :class_name => 'Address' enable_association_freezer #... end Now you can freeze the association at will: # in order def purchase #... freeze_billing_address freeze_shipping_address save! end When freezing an association, the attributes will be set to the frozen column. However, the model is not saved automatically, so you must call save at some point after freezing. Here's where the magic starts. Now when you fetch the frozen association (order.billing_address), the association freezer will step in and return an Address model which is built from the attributes in the frozen column, not the attributes in the addresses table. Therefore, any changes to the address will not effect the order's address. "freeze" is called on the model before returning it to prevent accidentally altering the data. You can also unfreeze an already frozen association if you want it to use the database record instead. unfreeze_billing_address You can then freeze it again to use the updated attributes. == Alternative Usage Not only is this gem good at handling the situation above, but it can also be used as a caching technique. If the belongs_to association is frequently accessed, it can be more efficient to freeze (cache) it to remove the need for a database call. Of course with this approach you want it to stay in sync with the database, so you'll need to expire the cache (refreeze the association) whenever the associated model changes. == Development This project can be found on github at the following URL. http://github.com/ryanb/association-freezer/ If you would like to contribute to this project, please fork the repository and send me a pull request.