How can I get an ActiveRecord::Association to initialize a parent's auto-loaded children's parent references to point at the actual parent object?

Question

This is in Ruby 1.9.3p194, with Rails 3.2.8

app/models/owner.rb:

class Owner < ActiveRecord::Base
  attr_accessible :name
  has_many :pets
end

app/models/pet.rb:

class Pet < ActiveRecord::Base
  attr_accessible :name, :owner
  belongs_to :owner
end

db/migrate/20120829184126_create_owners_and_pets.rb:

class CreateOwners < ActiveRecord::Migration
  def change
    create_table :owners do |t|
      t.string :name
      t.timestamps
    end
    create_table :pets do |t|
      t.string :name
      t.integer :owner_id
      t.timestamps
    end
  end
end

Alright, now watch what happens...

# rake db:migrate
# rails console
irb> shaggy = Owner.create(:name => 'Shaggy')
irb> shaggy.pets.build(:name => 'Scooby Doo')
irb> shaggy.pets.build(:name => 'Scrappy Doo')
irb> shaggy.object_id
  => 70262210740820
irb> shaggy.pets.map{|p| p.owner.object_id}
  => [70262210740820, 70262210740820]
irb> shaggy.name = 'Shaggie'
irb> shaggy.name
  => "Shaggie"
irb> shaggy.pets.map{|p| p.owner.name}
  => ["Shaggie", "Shaggie"]
irb> shaggy.save
irb> shaggy.reload
irb> shaggy.object_id
  => 70262210740820
irb> shaggy.pets.map{|p| p.owner.object_id}
  => [70262211070840, 70262211079640]
irb> shaggy.name = "Fred"
irb> shaggy.name
  => "Fred"
irb> shaggy.pets.map{|p| p.ower.name}
  => ["Shaggie", "Shaggie"]

My question: How can I get rails to initialize the elements of shaggy.pets to have their owners set to shaggy (the exact object), not only when the pet objects are first built/created, but even when they are auto-loaded from the database via the association?

Bonus points: Make it work in Rails 2.3.5 as well.

You can't, ActiveRecord isn't meant for this. You may want to look at DataMapper instead. — Tanzeeb Khalili, Aug 30 '12 at 04:24

score 1 · Answer 1 · answered Jan 16 '13 at 04:27

If you don't care about 2.3.5 support, the following is much simpler:

app/models/owner.rb:

class Owner < ActiveRecord::Base
  attr_accessible :name
  has_many :pets, :inverse_of => :owner
end

app/models/pet.rb:

class Pet < ActiveRecord::Base
  attr_accessible :name, :owner
  belongs_to :owner, :inverse_of => :pets
end

score 0 · Answer 2 · answered Aug 30 '12 at 05:01

Chris,

What you are missing is this;

When you query the database, you take a snapshot of the data in that exact moment.
Your associations do not point to the object reference, they are a shallow copy of the object queried from the database.
To make your code works, you need to save the object back to the database, so pets will appropriately fetch the updates.

I would also show more code, because I can clearly see you taking the wrong direction in what you're doing.

What you are showing is a desktop application pattern example, using patterns designed for web applications.

Chris · Accepted Answer · 2012-10-17T16:54:23.267

Figured it out. This works for me in Rails 3.2.8 and Rails 2.3.5:

class Owner < ActiveRecord::Base
  attr_accessible :name
  has_many :pets do
    def load_target
      super.map do |pet|
        pet.owner = (proxy_association.owner rescue proxy_owner)
        pet
      end
    end
  end
end

Note that in Rails 2.3.5, the object_ids of shaggy.pets.map{|p| p.owner} are still different, since they are different ProxyAssociation objects, but they at least point at the same underlying object.

If you do this often, you may want to generalize...

lib/remember_parent_extension.rb:

class RememberParentExtension < Module
  def initialize(parent_name)
    parent_setter = "#{parent_name}="
    super() do
      define_method(:load_target) do
        super.map do |child|
          parent_proxy = (proxy_association.owner rescue proxy_owner)
          child.send(parent_setter, parent_proxy)
          child
        end
      end
    end
  end
end

app/models/owner.rb:

class Owner < ActiveRecord::Base
  attr_accessible :name
  has_many :pets, :extend => RememberParentExtension.new(:owner)
end

How can I get an ActiveRecord::Association to initialize a parent's auto-loaded children's parent references to point at the actual parent object?

3 Answers3