diff --git a/fkreplace/__init__.py b/fkreplace/__init__.py
index e38d0c4..e8ded47 100644
--- a/fkreplace/__init__.py
+++ b/fkreplace/__init__.py
@@ -1,10 +1,21 @@
-# thanks to https://djangosnippets.org/snippets/2283/ for inspiration on m2m
 
-def migrate(from_obj, to_obj):
+def merge(from_obj, to_obj):
     for related in from_obj._meta.get_all_related_objects():
         accessor_name = related.get_accessor_name()
         varname = related.field.name
-        getattr(from_obj, accessor_name).all().update(**{varname: to_obj})
+        field = getattr(from_obj, accessor_name)
+        if related.multiple:
+            field.all().update(**{varname: to_obj})
+        elif related.one_to_one:
+            try:
+                getattr(to_obj, accessor_name)
+            except Exception as e:
+                # doesn't exist, safe to overwrite
+                setattr(field, varname, to_obj)
+                field.save()
+        else:
+            import pdb; pdb.set_trace()
+            raise Exception('unknown code path')
 
     for related_m2m 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 95e73fb..684954d 100644
--- a/tests/tests.py
+++ b/tests/tests.py
@@ -1,8 +1,39 @@
 from django.test import TestCase
-from .models import Person, Number, SSN
+from tests.models import Person, Number, SSN, Group
+from fkreplace import merge
 
-def setUp():
-    a = Person.objects.create(name='alf')
-    b = Person.objects.create(name='bee')
-    Number.objects.create(person=a, number='123')
-    Number.objects.create(person=a, number='1234')
+class MergeTests(TestCase):
+    def setUp(self):
+        self.a = Person.objects.create(name='alf')
+        self.b = Person.objects.create(name='bee')
+        self.c = Person.objects.create(name='sea')
+        Number.objects.create(person=self.a, number='555-1111')
+        Number.objects.create(person=self.a, number='555-1112')
+        Number.objects.create(person=self.b, number='555-1113')
+        SSN.objects.create(person=self.a, number='1')
+        SSN.objects.create(person=self.b, number='2')
+        self.g = Group.objects.create(name='Team Awesome')
+        self.g.people.add(self.a)
+        self.g.people.add(self.b)
+
+    def test_fk_simple(self):
+        merge(self.a, self.c)
+        # move FKs pointing at A to C
+        assert self.a.numbers.count() == 0
+        assert self.c.numbers.count() == 2
+        assert Number.objects.count() == 3
+
+    def test_fk_existing(self):
+        merge(self.a, self.b)
+        # everything now on b
+        assert self.a.numbers.count() == 0
+        assert self.b.numbers.count() == 3
+        assert Number.objects.count() == 3
+
+    def test_one2one_simple(self):
+        merge(self.a, self.c)
+        # move FKs pointing at A to C
+        #assert self.a.ssn is None
+        c = Person.objects.get(pk=self.c.pk)
+        c.ssn.number == 1
+        assert SSN.objects.count() == 2