Perf fix

If we're deleting all records in an association, don't add a IN(..)
clause to the query.

Fixes #3672.

activerecord/CHANGELOG.md

@@ -67,6 +67,11 @@
 
 ## Rails 3.1.3 (unreleased) ##
 
+*   Perf fix: If we're deleting all records in an association, don't add a IN(..) clause
+    to the query. *GH 3672*
+
+    *Jon Leighton*
+
 *   Fix bug with referencing other mysql databases in set_table_name. *GH 3690*
 
 *   Fix performance bug with mysql databases on a server with lots of other databses. *GH 3678*

activerecord/lib/active_record/associations/has_many_association.rb

@@ -89,8 +89,12 @@ module ActiveRecord
             records.each { |r| r.destroy }
             update_counter(-records.length) unless inverse_updates_counter_cache?
           else
-            keys  = records.map { |r| r[reflection.association_primary_key] }
-            scope = scoped.where(reflection.association_primary_key => keys)
+            scope = scoped
+
+            unless records == load_target
+              keys  = records.map { |r| r[reflection.association_primary_key] }
+              scope = scoped.where(reflection.association_primary_key => keys)
+            end
 
             if method == :delete_all
               update_counter(-scope.delete_all)