brand_identity.rb

class BrandIdentity < ActiveRecord::Base

  has_one :library_asset, :dependent => :destroy, :as => :asset

end

gistfile1.rb

brand_identity = BrandIdentity.new
brand_identity.build_library_asset
brand_identity.save!

# Validation error, asset must be present

brand_identity.library_asset
# => [Library Asset]

brand_identity.library_asset.asset
# => nil

# Why doesn't it set both sides of the relationship?

library_asset.rb

class LibraryAsset < ActiveRecord::Base

  belongs_to :asset, :polymorphic => true

  validates_presence_of :asset

end