Fix association with extension issues

This fixes the following issues.

* `association_scope` doesn't include `default_scope`. Should use `scope` instead.
* We can't use `method_missing` for customizing existing method.
* We can't use `relation_delegate_class` for sharing extensions. Should extend per association.

1 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/test/cases/associations/extension_test.rb b/activerecord/test/cases/associations/extension_test.rb
index 87d842f21d..f707a170f5 100644
--- a/activerecord/test/cases/associations/extension_test.rb
+++ b/activerecord/test/cases/associations/extension_test.rb
@@ -78,6 +78,12 @@ class AssociationsExtensionsTest < ActiveRecord::TestCase
     assert_equal post.association(:comments), post.comments.where("1=1").the_association
   end
 
+  def test_association_with_default_scope
+    assert_raises OopsError do
+      posts(:welcome).comments.destroy_all
+    end
+  end
+
   private
 
     def extend!(model)