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

activerecord/CHANGELOG

 *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]

activerecord/lib/active_record/associations.rb

@@ -584,7 +584,7 @@ def has_one(association_id, options = {})
         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

activerecord/test/associations_test.rb

@@ -291,6 +291,23 @@ def test_assignment_before_either_saved
     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