diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index b0c3a3179e196d7b0327be45c5f536a5189bef8d..9372adb583d2d0320f6b087c6bb4cc1171703e83 100644
--- a/activerecord/CHANGELOG
+++ b/activerecord/CHANGELOG
@@ -1,5 +1,7 @@
 *2.0.0 [Preview Release]* (September 29th, 2007) [Includes duplicates of changes from 1.14.2 - 1.15.3]
 
+* Make size for has_many :through use counter cache if it exists.  Closes #9734 [xaviershay]
+
 * Remove DB2 adapter since IBM chooses to maintain their own adapter instead.  [Jeremy Kemper]
 
 * Extract Oracle, SQLServer, and Sybase adapters into gems.  [Jeremy Kemper]
diff --git a/activerecord/lib/active_record/associations/has_many_through_association.rb b/activerecord/lib/active_record/associations/has_many_through_association.rb
index de0d7cc8a9a237f2b798477426e129ae91d64aa2..62839c46378f3076d0b1bc5f8b71b1d5eb8d94bb 100644
--- a/activerecord/lib/active_record/associations/has_many_through_association.rb
+++ b/activerecord/lib/active_record/associations/has_many_through_association.rb
@@ -100,7 +100,9 @@ def create!(attrs = nil)
       # calling collection.size if it has. If it's more likely than not that the collection does have a size larger than zero
       # and you need to fetch that collection afterwards, it'll take one less SELECT query if you use length.
       def size
-        loaded? ? @target.size : count
+        return @owner.send(:read_attribute, cached_counter_attribute_name) if has_cached_counter?
+        return @target.size if loaded?
+        return count
       end
 
       # Calculate sum using SQL, not Enumerable
@@ -258,6 +260,14 @@ def conditions
         end
 
         alias_method :sql_conditions, :conditions
+
+        def has_cached_counter?
+          @owner.attribute_present?(cached_counter_attribute_name)
+        end
+
+        def cached_counter_attribute_name
+          "#{@reflection.name}_count"
+        end
     end
   end
 end
diff --git a/activerecord/test/associations/join_model_test.rb b/activerecord/test/associations/join_model_test.rb
index d18c878733cd1eb42c1cf8cbc6c02b1ee981308d..d92a0c760a1783b9cafbe6f0086c051d434c5eda 100644
--- a/activerecord/test/associations/join_model_test.rb
+++ b/activerecord/test/associations/join_model_test.rb
@@ -457,6 +457,15 @@ def test_has_many_through_collection_size_doesnt_load_target_if_not_loaded
     assert !author.comments.loaded?
   end
 
+  uses_mocha('has_many_through_collection_size_uses_counter_cache_if_it_exists') do
+    def test_has_many_through_collection_size_uses_counter_cache_if_it_exists
+      author = authors(:david)
+      author.stubs(:read_attribute).with('comments_count').returns(100)
+      assert_equal 100, author.comments.size
+      assert !author.comments.loaded?
+    end
+  end
+
   def test_adding_junk_to_has_many_through_should_raise_type_mismatch
     assert_raise(ActiveRecord::AssociationTypeMismatch) { posts(:thinking).tags << "Uhh what now?" }
   end