Support Polymorphic Associated Objects: Take 2

lib/active_record/associated_object.rb

@@ -1,21 +1,23 @@
 class ActiveRecord::AssociatedObject
+  singleton_class.attr_reader :record_klass, :attribute_name
+  delegate :record_klass, :attribute_name, to: :class
+
   class << self
-    def inherited(klass)
-      record_klass   = klass.module_parent
-      record_name    = klass.module_parent_name.demodulize.underscore
-      attribute_name = klass.to_s.demodulize.underscore.to_sym
+    def polymorphic? = record_klass.nil?
 
-      unless record_klass.respond_to?(:descends_from_active_record?) && record_klass.descends_from_active_record?
-        raise ArgumentError, "#{record_klass} isn't valid; can only associate with ActiveRecord::Base subclasses"
-      end
+    def associate_with(record_klass)
+      raise ArgumentError, "#{record_klass} isn't valid; can only associate with ActiveRecord::Base subclasses" \
+        unless record_klass.respond_to?(:descends_from_active_record?) && record_klass.descends_from_active_record?
+
+      @record_klass   = record_klass
+      @attribute_name = to_s.demodulize.underscore.to_sym
 
-      klass.alias_method record_name, :record
-      klass.define_singleton_method(:record_klass)   { record_klass }
-      klass.define_singleton_method(:attribute_name) { attribute_name }
-      klass.delegate :record_klass, :attribute_name, to: :class
+      alias_method module_parent_name.demodulize.underscore, :record # `alias_method :post, :record` for a `Post::Publisher`.
     end
+    def inherited(klass) = klass.module_parent_name && klass.associate_with(klass.module_parent)
 
     def extension(&block)
+      raise ArgumentError, "calling `extension` on polymorphic Associated Objects isn't supported yet" if polymorphic?
       record_klass.class_eval(&block)
     end
 

test/active_record/associated_object_test.rb

@@ -143,4 +143,16 @@ class ActiveRecord::AssociatedObjectTest < ActiveSupport::TestCase
     assert_predicate Post::Comment.great.first, :rated_great?
     assert_match /test\/boot\/associated_object/, Post::Comment.instance_method(:rated_great?).source_location.first
   end
+
+  class PolymorphicTest < ActiveSupport::TestCase
+    test "polymorphic associated objects" do
+      post, comment = Post.first, Post::Comment.first
+
+      assert_kind_of Post::Pricing, post.pricing
+      assert_kind_of Post::Comment::Pricing, comment.pricing
+
+      assert post.pricing.post?
+      refute comment.pricing.post?
+    end
+  end
 end

test/boot/associated_object.rb

@@ -55,3 +55,16 @@ def great?
 end
 
 Post::Comment.has_object :rating
+
+# Can locate subclasses by grepping for `< Pricing`
+class Pricing < ActiveRecord::AssociatedObject
+  # Here, `record` will return the post for Post::Pricing objects and the comment for Post::Comment::Pricings.
+
+  def post? = record.is_a?(Post) # Probably don't do this in a real app.
+end
+
+class Post::Pricing < Pricing; end
+class Post::Comment::Pricing < Pricing; end
+
+Post.has_object :pricing
+Post::Comment.has_object :pricing