Merge pull request #4746 from jenslukowski/issue4718

Test and fix for issue 4718

1 files changed, 8 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/relation/finder_methods.rb b/activerecord/lib/active_record/relation/finder_methods.rb
index f1ac421a50..b54a167dc3 100644
--- a/activerecord/lib/active_record/relation/finder_methods.rb
+++ b/activerecord/lib/active_record/relation/finder_methods.rb
@@ -318,7 +318,8 @@ module ActiveRecord
     def find_one(id)
       id = id.id if ActiveRecord::Base === id
 
-      if IdentityMap.enabled? && where_values.blank? &&
+      if IdentityMap.enabled? &&
+        (where_values.blank? || contains_only_subclass_constraint?(where_values)) &&
         limit_value.blank? && order_values.blank? &&
         includes_values.blank? && preload_values.blank? &&
         readonly_value.nil? && joins_values.blank? &&
@@ -394,5 +395,11 @@ module ActiveRecord
     def using_limitable_reflections?(reflections)
       reflections.none? { |r| r.collection? }
     end
+    
+    def contains_only_subclass_constraint?(where_values)
+      where_values.length == 1 &&
+      !where_values[0].left.nil? && where_values[0].left.relation.name == table_name &&
+      !where_values[0].right.nil? && where_values[0].right.length == 1 && where_values[0].right[0] == @klass.name
+    end
   end
 end