ensure no self-merges

fkreplace/__init__.py

@@ -3,6 +3,9 @@ def merge(from_obj, to_obj):
     if not isinstance(from_obj, type(to_obj)):
         raise ValueError("both objects must be of the same type")
 
+    if from_obj.pk == to_obj.pk:
+        raise ValueError("cannot merge object with itself")
+
     for related in from_obj._meta.get_all_related_objects():
         accessor_name = related.get_accessor_name()
         varname = related.field.name

tests/tests.py

@@ -12,6 +12,11 @@ class MergeTests(TestCase):
         with self.assertRaises(ValueError):
             merge(self.a, self.g)
 
+    def test_same_object(self):
+        with self.assertRaises(ValueError):
+            # make sure this doesn't rely on is
+            merge(self.a, Person.objects.get(name=self.a.name))
+
     def test_fk_simple(self):
         Number.objects.create(person=self.a, number='555-1111')
         Number.objects.create(person=self.a, number='555-1112')