Don't save has_one associations unnecessarily. Closes #5735.

git-svn-id: http://svn-commit.rubyonrails.org/rails/trunk@4690 5ecf4fe2-1ee6-0310-87b1-e25e094e27de

3 files changed, 20 insertions, 1 deletions

diff --git a/activerecord/CHANGELOG b/activerecord/CHANGELOG
index 9b69dce47c..9e63b108b3 100644
--- a/activerecord/CHANGELOG
+++ b/activerecord/CHANGELOG
@@ -1,5 +1,7 @@
 *SVN*
 
+* Don't save has_one associations unnecessarily.  #5735 [Jonathan Viney]
+
 * Refactor ActiveRecord::Base.reset_subclasses to #reset, and add global observer resetting.  [Rick Olson]
 
 * Formally deprecate the deprecated finders. [Koz]
diff --git a/activerecord/lib/active_record/associations.rb b/activerecord/lib/active_record/associations.rb
index 192c7b7a5f..009153d1d2 100755
--- a/activerecord/lib/active_record/associations.rb
+++ b/activerecord/lib/active_record/associations.rb
@@ -584,7 +584,7 @@ module ActiveRecord
         module_eval do
           after_save <<-EOF
             association = instance_variable_get("@#{reflection.name}")
-            unless association.nil?
+            if !association.nil? && (new_record? || association.new_record? || association["#{reflection.primary_key_name}"] != id)
               association["#{reflection.primary_key_name}"] = id
               association.save(true)
             end
diff --git a/activerecord/test/associations_test.rb b/activerecord/test/associations_test.rb
index 747d04cf3f..eecb096521 100755
--- a/activerecord/test/associations_test.rb
+++ b/activerecord/test/associations_test.rb
@@ -291,6 +291,23 @@ class HasOneAssociationsTest < Test::Unit::TestCase
     assert_equal a, firm.account
     assert_equal a, firm.account(true)
   end
+
+  def test_not_resaved_when_unchanged
+    firm = Firm.find(:first, :include => :account)
+    assert_queries(1) { firm.save! }
+
+    firm = Firm.find(:first)
+    firm.account = Account.find(:first)
+    assert_queries(1) { firm.save! }
+
+    firm = Firm.find(:first).clone
+    firm.account = Account.find(:first)
+    assert_queries(2) { firm.save! }
+
+    firm = Firm.find(:first).clone
+    firm.account = Account.find(:first).clone
+    assert_queries(2) { firm.save! }
+  end
 end