Fix #3672 again (dependent: delete_all perf)

activerecord/lib/active_record/associations/builder/has_and_belongs_to_many.rb

@@ -18,7 +18,7 @@ module ActiveRecord::Associations::Builder
         model.send(:include, Module.new {
           class_eval <<-RUBY, __FILE__, __LINE__ + 1
             def destroy_associations
-              association(#{name.to_sym.inspect}).delete_all
+              association(#{name.to_sym.inspect}).delete_all_on_destroy
               super
             end
           RUBY

activerecord/lib/active_record/associations/builder/has_many.rb

@@ -44,12 +44,18 @@ module ActiveRecord::Associations::Builder
       end
 
       def define_delete_all_dependency_method
+        name = self.name
+        mixin.redefine_method(dependency_method_name) do
+          association(name).delete_all_on_destroy
+        end
+      end
+
+      def define_nullify_dependency_method
         name = self.name
         mixin.redefine_method(dependency_method_name) do
           send(name).delete_all
         end
       end
-      alias :define_nullify_dependency_method :define_delete_all_dependency_method
 
       def define_restrict_dependency_method
         name = self.name

activerecord/lib/active_record/associations/collection_association.rb

@@ -152,6 +152,13 @@ module ActiveRecord
         end
       end
 
+      # Called when the association is declared as :dependent => :delete_all. This is
+      # an optimised version which avoids loading the records into memory. Not really
+      # for public consumption.
+      def delete_all_on_destroy
+        scoped.delete_all
+      end
+
       # Destroy all the records from this association.
       #
       # See destroy for more info.

activerecord/lib/active_record/associations/has_and_belongs_to_many_association.rb

@@ -32,6 +32,10 @@ module ActiveRecord
         record
       end
 
+      # ActiveRecord::Relation#delete_all needs to support joins before we can use a
+      # SQL-only implementation.
+      alias delete_all_on_destroy delete_all
+
       private
 
         def count_records

activerecord/lib/active_record/associations/has_many_association.rb

@@ -89,12 +89,8 @@ module ActiveRecord
             records.each { |r| r.destroy }
             update_counter(-records.length) unless inverse_updates_counter_cache?
           else
-            scope = scoped
-
-            unless records == load_target
-              keys  = records.map { |r| r[reflection.association_primary_key] }
-              scope = scoped.where(reflection.association_primary_key => keys)
-            end
+            keys  = records.map { |r| r[reflection.association_primary_key] }
+            scope = scoped.where(reflection.association_primary_key => keys)
 
             if method == :delete_all
               update_counter(-scope.delete_all)
@@ -103,7 +99,7 @@ module ActiveRecord
             end
           end
         end
-        
+
         def foreign_key_present?
           owner.attribute_present?(reflection.association_primary_key)
         end

activerecord/lib/active_record/associations/has_many_through_association.rb

@@ -54,6 +54,10 @@ module ActiveRecord
         record
       end
 
+      # ActiveRecord::Relation#delete_all needs to support joins before we can use a
+      # SQL-only implementation.
+      alias delete_all_on_destroy delete_all
+
       private
 
         def through_association