How best to associate an Address to multiple models in rails?

Question

This question on SO appears to be related to my question, but I\'m not sure my question is answered by that.

An address can belong to more than one model (UserProfile an

Answer 1

You missed one option: have a class that contains the common behavior and add the fields to all tables. Use a composed_of aggregate to manage the data.

class Address
  attr_accessor :line1, :line2, :city, :state, :zip

  def initialize(line1, line2, city, state, zip)
    @line1 = line1
  end
end

class UserProfile < ActiveRecord::Base
  composed_of :address, :mapping => %w(line1 line2 city state zip)
end

class Event < ActiveRecord::Base
  composed_of :address, :mapping => %w(line1 line2 city state zip)
end

See #composed_of in the Ruby on Rails API docs.

Answer 2

One way would be standard Rails polymorphism:

class Address
  belongs_to :addressable, :polymorphic => true
end

class UserProfile
  has_one address, :as => :addressable
end

class Event
  has_one address, :as => :addressable
end

It could be the nagging feeling you have about this is that you can't create a db-level constraint with Rails-style polymorphic relationships. The alternative (suggested by Dan Chak in Enterprise Rails) is like your #1 option, where you do indeed create a separate id field for each type of relationship. This does leave unused fields, but it also allows for constraints. I can see the argument for both, but the Rails community has been using AR polymorphism for some time with apparently good success. I use it without hesitation. But if that bugs you, you can use Chak's approach. It's a lot more work. : )

EDIT: @Slick86, migration would look something like:

class CreateAddresses < ActiveRecord::Migration
  def change
    create_table :addresses do |t|
      t.integer :addressable_id
      t.string :addressable_type
    end
  end
end