== Overview

This PersistedVirtualAttributes plugin persists virtual attributes (hey, that sounds like a good plugin name!) in a specified text column in the database. Alternatively, you could think of it as making the mechanics of managing serialized data for a model transparent to the user, so rather than:

model.serialized_column = {} model.serialized_column[:food] => :pickles

you can simply do:

model.food = :pickles

== Expectations The plugin requires the existence of a text column on the model it's applied to, unused by anything else in the application (it'll be serialized as a hash and used to store the virtual attributes).

Note that only objects which can be serialized by Rails can be persisted.

== Motivation / Example uses

Consider an application which verifies user identities through various ID verification services. Each remote service asks its own set of questions (some want Driver's License number and State, others want the user's address, etc.) and returns its own custom information (e.g. a yes/no boolean, or a confidence interval, or custom errors).

Each verification service is represented by a separate model. One approach would be to create a new database table for each model, but that's kinda ugly (cluttering the DB unneccessarily) and the models share so much in common that they're already STI subclasses off a general Verification model anyway.

Assuming the actual verified/not verified outcome is cached in a boolean column on the User model, the Verification model itself will only rarely be read out of the DB (probably just by admins reviewing verification history) so adding two serialized text columns isn't a performance concern. Enter PersistedVirtualAttributes:

class Verification < ActiveRecord::Base

... shared verification stuff here

end

class AddressVerification < Verification persist_virtual_attributes :in => :custom_data, :attributes => [:street, :city, :state, :zip]

persist_virtual_attributes :in => :response_data, :attributes => [:response_xml, :verification_status]

validates_presence_of :street, :city, :state

end

class DriversLicenseVerification < Verification persist_virtual_attributes :store_column => :custom_data, # Same thing as :in, in case you want to be explicit :attributes => [:number, :expiration, :state]

persist_virtual_attributes :in => :response_data,
                          :attributes => [:response_json, :verification_status]

validates_presence_of :state, :number, :expiration
validates_numericality_of :number

end

Now you can do things like:

a = AddressVerification.new(:street => '123 Main St.', :city => 'San Francisco') a.state = 'CA' a.city # => 'San Francisco' a.save # => true b = AddressVerification.first b.street # => '123 Main St.'

Note that you can run validations on the virtual attributes just like you can with real columns. The only difference is that when the model is stored the virtual attributes are serialized to a single text column so each different subclass can easily have its own custom attributes without messing with the DB schema. Glorious DRYness.

Copyright (c) 2009 Kali Donovan, released under the MIT license