diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index dc61ab3c31..bd06810c36 100644
--- a/activerecord/CHANGELOG
+++ b/activerecord/CHANGELOG
@@ -1,5 +1,13 @@
 *SVN*
 
+* Added support for eagerly including polymorphic has_one associations. (closes #4525) [Rick]
+
+    class Post < ActiveRecord::Base
+      has_one :tagging, :as => :taggable
+    end
+    
+    Post.find :all, :include => :tagging
+
 * Added descriptive error messages for invalid has_many :through associations: going through :has_one or :has_and_belongs_to_many [Rick]
 
 * Added support for going through a polymorphic has_many association: (closes #4401) [Rick]
diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 5319fa39e8..7b6257ec03 100755
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -1392,7 +1392,7 @@ module ActiveRecord
               @aliased_prefix     = "t#{ join_dependency.joins.size }"
               @aliased_table_name = table_name # start with the table name
               @parent_table_name  = parent.active_record.table_name
-              
+
               if !parent.table_joins.blank? && parent.table_joins.to_s.downcase =~ %r{join(\s+\w+)?\s+#{aliased_table_name.downcase}\son}
                 join_dependency.table_aliases[aliased_table_name] += 1
               end
@@ -1473,12 +1473,16 @@ module ActiveRecord
                         aliased_table_name, "#{reflection.options[:as]}_type",
                         klass.quote(parent.active_record.base_class.name)
                       ]
-                      
+                    when reflection.macro == :has_one && reflection.options[:as]
+                      " LEFT OUTER JOIN %s ON %s.%s = %s.%s AND %s.%s = %s " % [
+                        table_name_and_alias,
+                        aliased_table_name, "#{reflection.options[:as]}_id",
+                        parent.aliased_table_name, parent.primary_key,
+                        aliased_table_name, "#{reflection.options[:as]}_type",
+                        klass.quote(reflection.active_record.name)
+                      ]
                     else
-                      foreign_key = options[:foreign_key] || case reflection.macro
-                        when :has_many then reflection.active_record.to_s.classify
-                        when :has_one  then reflection.active_record.to_s
-                      end.foreign_key
+                      foreign_key = options[:foreign_key] || reflection.active_record.name.foreign_key
                       " LEFT OUTER JOIN %s ON %s.%s = %s.%s " % [
                         table_name_and_alias,
                         aliased_table_name, foreign_key,
diff --git a/activerecord/test/associations_join_model_test.rb b/activerecord/test/associations_join_model_test.rb
index b7f0af173b..4a11d4c5eb 100644
--- a/activerecord/test/associations_join_model_test.rb
+++ b/activerecord/test/associations_join_model_test.rb
@@ -208,6 +208,14 @@ class AssociationsJoinModelTest < Test::Unit::TestCase
     end
   end
 
+  def test_include_polymorphic_has_one
+    post    = Post.find_by_id(posts(:welcome).id, :include => :tagging)
+    tagging = taggings(:welcome_general)
+    assert_no_queries do
+      assert_equal tagging, post.tagging
+    end
+  end
+
   def test_include_polymorphic_has_many_through
     posts           = Post.find(:all, :order => 'posts.id')
     posts_with_tags = Post.find(:all, :include => :tags, :order => 'posts.id')
@@ -303,6 +311,14 @@ class AssociationsJoinModelTest < Test::Unit::TestCase
     assert_equal [taggings(:welcome_general), taggings(:thinking_general)], authors(:david).taggings.uniq.sort_by { |t| t.id }
   end
 
+  def test_has_many_through_polymorphic_has_many_with_eager_loading
+    author            = Author.find_by_id(authors(:david).id, :include => :taggings)
+    expected_taggings = [taggings(:welcome_general), taggings(:thinking_general)]
+    assert_no_queries do
+      assert_equal expected_taggings, author.taggings.uniq.sort_by { |t| t.id }
+    end
+  end
+
   def test_has_many_through_has_many_through
     assert_raise(ActiveRecord::HasManyThroughSourceAssociationMacroError) { authors(:david).tags }
   end
diff --git a/activerecord/test/fixtures/tag.rb b/activerecord/test/fixtures/tag.rb
index ed3c6ce34a..c12ec0c188 100644
--- a/activerecord/test/fixtures/tag.rb
+++ b/activerecord/test/fixtures/tag.rb
@@ -1,5 +1,5 @@
 class Tag < ActiveRecord::Base
-  has_many :taggings,  :as => :taggable
+  has_many :taggings
   has_many :taggables, :through => :taggings
-  has_one  :tagging,   :as => :taggable
+  has_one  :tagging
 end
\ No newline at end of file