Index: django/db/models/fields/related.py
===================================================================
--- django/db/models/fields/related.py	(revision 7968)
+++ django/db/models/fields/related.py	(working copy)
@@ -134,12 +134,16 @@
             # that object. In certain conditions (especially one-to-one relations),
             # the primary key may itself be an object - so we need to keep drilling
             # down until we hit a value that can be used for a comparison.
-            v = value
+            v, field = value, None
             try:
                 while True:
-                    v = getattr(v, v._meta.pk.name)
+                    v, field = getattr(v, v._meta.pk.name), v._meta.pk
             except AttributeError:
                 pass
+            if field:
+                v = field.get_db_prep_lookup(lookup_type, v)
+                if isinstance(v, list):
+                    v = v[0]
             return v
 
         if hasattr(value, 'as_sql'):