use a hash for caching aggregations rather than ivars

activerecord/lib/active_record/aggregations.rb

@@ -4,9 +4,7 @@ module ActiveRecord
     extend ActiveSupport::Concern
 
     def clear_aggregation_cache #:nodoc:
-      self.class.reflect_on_all_aggregations.to_a.each do |assoc|
-        instance_variable_set "@#{assoc.name}", nil
-      end if self.persisted?
+      @aggregation_cache.clear if persisted?
     end
 
     # Active Record implements aggregation through a macro-like class method called +composed_of+
@@ -224,11 +222,7 @@ module ActiveRecord
         def reader_method(name, class_name, mapping, allow_nil, constructor)
           module_eval do
             define_method(name) do
-              unless instance_variable_defined?("@#{name}")
-                instance_variable_set("@#{name}", nil)
-              end
-
-              if (instance_variable_get("@#{name}").nil?) && (!allow_nil || mapping.any? {|pair| !read_attribute(pair.first).nil? })
+              if (@aggregation_cache[name].nil?) && (!allow_nil || mapping.any? {|pair| !read_attribute(pair.first).nil? })
                 attrs = mapping.collect {|pair| read_attribute(pair.first)}
                 object = case constructor
                   when Symbol
@@ -238,9 +232,9 @@ module ActiveRecord
                   else
                     raise ArgumentError, 'Constructor must be a symbol denoting the constructor method to call or a Proc to be invoked.'
                   end
-                instance_variable_set("@#{name}", object)
+                @aggregation_cache[name] = object
               end
-              instance_variable_get("@#{name}")
+              @aggregation_cache[name]
             end
           end
 
@@ -251,7 +245,7 @@ module ActiveRecord
             define_method("#{name}=") do |part|
               if part.nil? && allow_nil
                 mapping.each { |pair| self[pair.first] = nil }
-                instance_variable_set("@#{name}", nil)
+                @aggregation_cache[name] = nil
               else
                 unless part.is_a?(class_name.constantize) || converter.nil?
                   part = case converter
@@ -265,7 +259,7 @@ module ActiveRecord
                 end
 
                 mapping.each { |pair| self[pair.first] = part.send(pair.last) }
-                instance_variable_set("@#{name}", part.freeze)
+                @aggregation_cache[name] = part.freeze
               end
             end
           end

activerecord/lib/active_record/base.rb

@@ -1369,6 +1369,7 @@ MSG
       def initialize(attributes = nil)
         @attributes = attributes_from_column_definition
         @association_cache = {}
+        @aggregation_cache = {}
         @attributes_cache = {}
         @new_record = true
         @readonly = false
@@ -1417,6 +1418,7 @@ MSG
         @attributes = coder['attributes']
         @attributes_cache, @previously_changed, @changed_attributes = {}, {}, {}
         @association_cache = {}
+        @aggregation_cache = {}
         @readonly = @destroyed = @marked_for_destruction = false
         @new_record = false
         _run_find_callbacks
@@ -1628,8 +1630,7 @@ MSG
           @changed_attributes[attr] = orig_value if field_changed?(attr, orig_value, @attributes[attr])
         end
 
-        clear_aggregation_cache
-
+        @aggregation_cache = {}
         @association_cache = {}
         @attributes_cache = {}
         @new_record  = true