diff --git a/django/core/meta/fields.py b/django/core/meta/fields.py
index da201ef8f3..56ded6edd7 100644
--- a/django/core/meta/fields.py
+++ b/django/core/meta/fields.py
@@ -36,8 +36,12 @@ def manipulator_valid_rel_key(f, self, field_data, all_data):
 
 def manipulator_validator_unique(f, opts, self, field_data, all_data):
     "Validates that the value is unique for this field."
+    if f.rel and isinstance(f.rel, ManyToOne):
+        lookup_type = 'pk'
+    else:
+        lookup_type = 'exact'
     try:
-        old_obj = opts.get_model_module().get_object(**{'%s__exact' % f.name: field_data})
+        old_obj = opts.get_model_module().get_object(**{'%s__%s' % (f.name, lookup_type): field_data})
     except ObjectDoesNotExist:
         return
     if hasattr(self, 'original_object') and getattr(self.original_object, opts.pk.column) == getattr(old_obj, opts.pk.column):