Rails Polymorphic Association with multiple associations on the same model

My question is essentially the same as this one: Polymorphic Association with multiple associations on the same model

However, the proposed/accepted solution does not work, as illustrated by a commenter later.

I have a Photo class that is used all over my app. A post can have a single photo. However, I want to re-use the polymorphic relationship to add a secondary photo.

Before:

class Photo 
   belongs_to :attachable, :polymorphic => true
end

class Post
   has_one :photo, :as => :attachable, :dependent => :destroy
end

Desired:

class Photo 
   belongs_to :attachable, :polymorphic => true
end

class Post
   has_one :photo,           :as => :attachable, :dependent => :destroy
   has_one :secondary_photo, :as => :attachable, :dependent => :destroy
end

However, this fails as it cannot find the class "SecondaryPhoto". Based on what I could tell from that other thread, I'd want to do:

   has_one :secondary_photo, :as => :attachable, :class_name => "Photo", :dependent => :destroy

Except calling Post#secondary_photo simply returns the same photo that is attached via the Photo association, e.g. Post#photo === Post#secondary_photo. Looking at the SQL, it does WHERE type = "Photo" instead of, say, "SecondaryPhoto" as I'd like...

Thoughts? Thanks!

Solution 1:

I have done that in my project.

The trick is that photos need a column that will be used in has_one condition to distinguish between primary and secondary photos. Pay attention to what happens in :conditions here.

has_one :photo, :as => 'attachable', 
        :conditions => {:photo_type => 'primary_photo'}, :dependent => :destroy

has_one :secondary_photo, :class_name => 'Photo', :as => 'attachable',
        :conditions => {:photo_type => 'secondary_photo'}, :dependent => :destroy

The beauty of this approach is that when you create photos using @post.build_photo, the photo_type will automatically be pre-populated with corresponding type, like 'primary_photo'. ActiveRecord is smart enough to do that.

Solution 2:

In Rails 5 you have to define attr_accessor for :attachable_id and specify for relation :class_name and :foreign_key options only. You will get ...AND attachable_type = 'SecondaryPhoto' if as: :attachable used

class Post
  attr_accessor :attachable_id
  has_one :photo, :as => :attachable, :dependent => :destroy
  has_one :secondary_photo, -> { where attachable_type: 'SecondaryPhoto' }, class_name: "Photo", dependent: :destroy, foreign_key: :attachable_id

Rails 4.2+

class Photo
   belongs_to :attachable, :polymorphic => true
end

class Post
   has_one :photo, :as => :attachable, :dependent => :destroy
   has_one :secondary_photo, -> { where attachable_type: "SecondaryPhoto"},
     class_name: Photo, foreign_key: :attachable_id,
     foreign_type: :attachable_type, dependent: :destroy
end

You need to provide foreign_key according ....able'ness or Rails will ask for post_id column in photo table. Attachable_type column will fills with Rails magic as SecondaryPhoto

Solution 3:

None of the previous answers helped me solve this problem, so I'll put this here incase anyone else runs into this. Using Rails 4.2 +.

Create the migration (assuming you have an Addresses table already):

class AddPolymorphicColumnsToAddress < ActiveRecord::Migration
  def change
    add_column :addresses, :addressable_type, :string, index: true
    add_column :addresses, :addressable_id, :integer, index: true
    add_column :addresses, :addressable_scope, :string, index: true
  end
end

Setup your polymorphic association:

class Address < ActiveRecord::Base
  belongs_to :addressable, polymorphic: true
end

Setup the class where the association will be called from:

class Order < ActiveRecord::Base
  has_one :bill_address, -> { where(addressable_scope: :bill_address) }, as: :addressable,  class_name: "Address", dependent: :destroy
  accepts_nested_attributes_for :bill_address, allow_destroy: true

  has_one :ship_address, -> { where(addressable_scope: :ship_address) }, as: :addressable, class_name: "Address", dependent: :destroy
  accepts_nested_attributes_for :ship_address, allow_destroy: true
end

The trick is that you have to call the build method on the Order instance or the scope column won't be populated.

So this does NOT work:

address = {attr1: "value"... etc...}
order = Order.new(bill_address: address)
order.save!

However, this DOES WORK.

address = {attr1: "value"... etc...}
order = Order.new
order.build_bill_address(address)
order.save!

Hope that helps someone else.