Avoid empty transaction from setting has_one association on new record.

activerecord/CHANGELOG.md

@@ -1,3 +1,8 @@
+*   Setting a has_one association on a new record no longer causes an empty
+    transaction.
+
+    *Dylan Thacker-Smith*
+
 *   Re-use `order` argument pre-processing for `reorder`.
 
     *Paul Nikitochkin*

activerecord/lib/active_record/associations/has_one_association.rb

@@ -27,6 +27,7 @@ module ActiveRecord
 
         return self.target if !(target || record)
         if (target != record) || record.changed?
+          save &&= owner.persisted?
           transaction_if(save) do
             remove_target!(options[:dependent]) if target && !target.destroyed?
 
@@ -34,7 +35,7 @@ module ActiveRecord
               set_owner_attributes(record)
               set_inverse_instance(record)
 
-              if owner.persisted? && save && !record.save
+              if save && !record.save
                 nullify_owner_attributes(record)
                 set_owner_attributes(target) if target
                 raise RecordNotSaved, "Failed to save the new associated #{reflection.name}."

activerecord/test/cases/associations/has_one_associations_test.rb

@@ -505,6 +505,8 @@ class HasOneAssociationsTest < ActiveRecord::TestCase
     assert_no_queries { company.account = nil }
     account = Account.find(2)
     assert_queries { company.account = account }
+
+    assert_no_queries { Firm.new.account = account }
   end
 
   def test_has_one_assignment_triggers_save_on_change