Modify validity check from r16678 slightly to work with Oracle.

We now skip this particular check on Oracle backends. Change is based on
a backend feature check, so it also works with other backends that may
treat NULL as equal to an empty string.

Fixes #16694 (with luck).

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16681 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/core/management/validation.py

@@ -39,7 +39,11 @@ def get_validation_errors(outfile, app=None):
                 e.add(opts, '"%s": You can\'t use "id" as a field name, because each model automatically gets an "id" field if none of the fields have primary_key=True. You need to either remove/rename your "id" field or add primary_key=True to a field.' % f.name)
             if f.name.endswith('_'):
                 e.add(opts, '"%s": Field names cannot end with underscores, because this would lead to ambiguous queryset filters.' % f.name)
-            if f.primary_key and f.null:
+            if (f.primary_key and f.null and
+                    not connection.features.interprets_empty_strings_as_nulls):
+                # We cannot reliably check this for backends like Oracle which
+                # consider NULL and '' to be equal (and thus set up
+                # character-based fields a little differently).
                 e.add(opts, '"%s": Primary key fields cannot have null=True.' % f.name)
             if isinstance(f, models.CharField):
                 try: