diff --git a/activerecord/lib/active_record/associations/collection_association.rb b/activerecord/lib/active_record/associations/collection_association.rb
index 17795f1b7d..c32dd77420 100644
--- a/activerecord/lib/active_record/associations/collection_association.rb
+++ b/activerecord/lib/active_record/associations/collection_association.rb
@@ -402,6 +402,9 @@ def merge_target_lists(persisted, memory)
           return memory    if persisted.empty?
 
           persisted.map! do |record|
+
+            # To work with ruby 1.8.7
+            # > 1.9 #=> mem_record = memory.delete(record)
             mem_record_index = memory.index(record)
             if mem_record_index
               mem_record = memory.at(mem_record_index)