Memoize association.named_scope calls

activerecord/lib/active_record/associations/association_collection.rb

@@ -405,6 +405,9 @@ def method_missing(method, *args)
             else
               super
             end
+          elsif @reflection.klass.scopes[method]
+            @_scopes ||= {}
+            @_scopes[method] ||= with_scope(construct_scope) { @reflection.klass.send(method, *args) }
           else
             with_scope(construct_scope) do
               if block_given?

activerecord/test/cases/named_scope_test.rb

@@ -413,6 +413,15 @@ def test_nested_named_scopes_queries_size
       Topic.approved.by_lifo.replied.written_before(Time.now).all
     end
   end
+
+  def test_named_scopes_are_cached_on_associations
+    post = posts(:welcome)
+
+    assert_equal post.comments.containing_the_letter_e.object_id, post.comments.containing_the_letter_e.object_id
+
+    post.comments.containing_the_letter_e.all # force load
+    assert_no_queries { post.comments.containing_the_letter_e.all }
+  end
 end
 
 class DynamicScopeMatchTest < ActiveRecord::TestCase