Don't remove the target if it has already been destroyed

activerecord/lib/active_record/associations/has_one_association.rb

@@ -10,7 +10,7 @@ def replace(record, save = true)
 
         reflection.klass.transaction do
           if target && target != record
-            remove_target!(options[:dependent])
+            remove_target!(options[:dependent]) unless target.destroyed?
           end
 
           if record

activerecord/test/cases/associations/has_one_associations_test.rb

@@ -95,6 +95,15 @@ def test_nullification_on_association_change
     assert_nil Account.find(old_account_id).firm_id
   end
 
+  def test_natural_assignment_to_nil_after_destroy
+    firm = companies(:rails_core)
+    old_account_id = firm.account.id
+    firm.account.destroy
+    firm.account = nil
+    assert_nil companies(:rails_core).account
+    assert_raise(ActiveRecord::RecordNotFound) { Account.find(old_account_id) }
+  end
+
   def test_association_change_calls_delete
     companies(:first_firm).deletable_account = Account.new(:credit_limit => 5)
     assert_equal [], Account.destroyed_account_ids[companies(:first_firm).id]