diff --git a/fkreplace/__init__.py b/fkreplace/__init__.py
index 0b554ef..7494cc3 100644
--- a/fkreplace/__init__.py
+++ b/fkreplace/__init__.py
@@ -1,5 +1,20 @@
+from django.core.exceptions import ObjectDoesNotExist
 
-def merge(from_obj, to_obj):
+
+class MergeException(Exception):
+    pass
+
+
+class OneToOneConflict(MergeException):
+    pass
+
+
+ERROR = 0
+KEEP = 1
+UPDATE = 2
+
+
+def merge(from_obj, to_obj, one_to_one_conflict=ERROR):
     if not isinstance(from_obj, type(to_obj)):
         raise ValueError("both objects must be of the same type")
 
@@ -17,17 +32,17 @@ def merge(from_obj, to_obj):
             try:
                 field = getattr(from_obj, accessor_name)
                 try:
-                    getattr(to_obj, accessor_name)
-                except Exception as e:
+                    to_obj_field = getattr(to_obj, accessor_name)
+                    raise OneToOneConflict("both fields have an attribute set for {}".format(accessor_name))
+                except ObjectDoesNotExist:
                     # doesn't exist, safe to overwrite
                     setattr(field, varname, to_obj)
                     field.save()
-            except Exception as e:
+            except ObjectDoesNotExist:
                 # from_obj one to one isn't set, skip
                 pass
         else:
-            import pdb; pdb.set_trace()
-            raise Exception('unknown code path')
+            raise NotImplementedError('unexpected relation type, please file a bug')
 
     for related in from_obj._meta.get_all_related_many_to_many_objects():
         accessor_name = related.get_accessor_name()
diff --git a/tests/tests.py b/tests/tests.py
index dfc0efd..3e2499c 100644
--- a/tests/tests.py
+++ b/tests/tests.py
@@ -1,6 +1,6 @@
 from django.test import TestCase
 from .models import Person, Number, SSN, Group
-from fkreplace import merge
+from fkreplace import merge, OneToOneConflict
 
 class MergeTests(TestCase):
     def setUp(self):
@@ -44,6 +44,12 @@ class MergeTests(TestCase):
         assert SSN.objects.count() == 1
 
     # TODO: test one2one when there's a conflict
+    def test_one2one_conflict(self):
+        SSN.objects.create(person=self.a, number='1')
+        SSN.objects.create(person=self.b, number='1')
+
+        with self.assertRaises(OneToOneConflict):
+            merge(self.a, self.b)
 
     def test_many2many_simple(self):
         self.g.people.add(self.a)