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.

2 files changed, 15 insertions, 1 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)
diff --git a/activerecord/test/cases/associations/has_many_associations_test.rb b/activerecord/test/cases/associations/has_many_associations_test.rb
index 6a479a344c..a794eba691 100644
--- a/activerecord/test/cases/associations/has_many_associations_test.rb
+++ b/activerecord/test/cases/associations/has_many_associations_test.rb
@@ -2251,7 +2251,15 @@ class HasManyAssociationsTest < ActiveRecord::TestCase
   test "association with extend option with multiple extensions" do
     post = posts(:welcome)
     assert_equal "lifo",  post.comments_with_extend_2.author
-    assert_equal "hello", post.comments_with_extend_2.greeting
+    assert_equal "hullo", post.comments_with_extend_2.greeting
+  end
+
+  test "extend option affects per association" do
+    post = posts(:welcome)
+    assert_equal "lifo",  post.comments_with_extend.author
+    assert_equal "lifo",  post.comments_with_extend_2.author
+    assert_equal "hello", post.comments_with_extend.greeting
+    assert_equal "hullo", post.comments_with_extend_2.greeting
   end
 
   test "delete record with complex joins" do