Ensure that has_many :through associations use a count query instead of loading the target when #size is called. Closes #8800 [lifo]

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@7237 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

3 files changed, 15 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index 5cb626f91a..4adcfcc4df 100644
--- a/activerecord/CHANGELOG
+++ b/activerecord/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+# Ensure that has_many :through associations use a count query instead of loading the target when #size is called.  Closes #8800 [lifo]
+
 * Added :unless clause to validations #8003 [monki]. Example: 
 
     def using_open_id?
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 ebe1be04e5..ec093548da 100644
--- a/activerecord/lib/active_record/associations/has_many_through_association.rb
+++ b/activerecord/lib/active_record/associations/has_many_through_association.rb
@@ -96,6 +96,13 @@ module ActiveRecord
         end
       end
 
+      # Returns the size of the collection by executing a SELECT COUNT(*) query if the collection hasn't been loaded and
+      # 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
+      end
+
       # Calculate sum using SQL, not Enumerable
       def sum(*args, &block)
         calculate(:sum, *args, &block)
diff --git a/activerecord/test/associations/join_model_test.rb b/activerecord/test/associations/join_model_test.rb
index a24763b92e..d18c878733 100644
--- a/activerecord/test/associations/join_model_test.rb
+++ b/activerecord/test/associations/join_model_test.rb
@@ -451,6 +451,12 @@ class AssociationsJoinModelTest < Test::Unit::TestCase
     assert_nothing_raised { vertices(:vertex_1).sinks << vertices(:vertex_5) }
   end
 
+  def test_has_many_through_collection_size_doesnt_load_target_if_not_loaded
+    author = authors(:david)
+    assert_equal 9, author.comments.size
+    assert !author.comments.loaded?
+  end
+
   def test_adding_junk_to_has_many_through_should_raise_type_mismatch
     assert_raise(ActiveRecord::AssociationTypeMismatch) { posts(:thinking).tags << "Uhh what now?" }
   end