Optimize <association>_ids for hm:t with belongs_to source

1 files changed, 8 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 10ecd068d3..1b884fd2ab 100755
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -1273,9 +1273,16 @@ module ActiveRecord
             if send(reflection.name).loaded? || reflection.options[:finder_sql]
               send(reflection.name).map(&:id)
             else
-              send(reflection.name).all(:select => "#{reflection.quoted_table_name}.#{reflection.klass.primary_key}").map(&:id)
+              if reflection.through_reflection && reflection.source_reflection.belongs_to?
+                through = reflection.through_reflection
+                primary_key = reflection.source_reflection.primary_key_name
+                send(through.name).all(:select => "DISTINCT #{through.quoted_table_name}.#{primary_key}").map(&:"#{primary_key}")
+              else
+                send(reflection.name).all(:select => "#{reflection.quoted_table_name}.#{reflection.klass.primary_key}").map(&:id)
+              end
             end
           end
+
         end
 
         def collection_accessor_methods(reflection, association_proxy_class, writer = true)