Fixed #2108 -- allow saving of empty models, rather than just dropping them.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@3115 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/backends/postgresql_psycopg2/base.py

@@ -114,6 +114,9 @@ def get_fulltext_search_sql(field_name):
 def get_drop_foreignkey_sql():
     return "DROP CONSTRAINT"
 
+def get_pk_default_value():
+    return "DEFAULT"
+
 OPERATOR_MAPPING = {
     'exact': '= %s',
     'iexact': 'ILIKE %s',