add an xfail test for what we can't fix

Author: Changaco

tests.py

@@ -8,7 +8,7 @@
 from postgres.cursors import TooFew, TooMany, SimpleDictCursor
 from postgres.orm import ReadOnly, Model
 from psycopg2 import InterfaceError, ProgrammingError
-from pytest import raises
+from pytest import mark, raises
 
 
 DATABASE_URL = os.environ['DATABASE_URL']
@@ -339,6 +339,14 @@ def test_add_column_doesnt_break_anything(self):
         one = self.db.one("SELECT foo.*::foo FROM foo WHERE bar='baz'")
         assert one.boo is None
 
+    @mark.xfail
+    def test_add_and_drop_columns(self):
+        self.db.run("ALTER TABLE foo ADD COLUMN biz int NOT NULL DEFAULT 0")
+        self.db.run("ALTER TABLE foo DROP COLUMN bar")
+        one = self.db.one("SELECT foo.*::foo FROM foo LIMIT 1")
+        assert one.biz == 0
+        assert not hasattr(one, 'bar')
+
 
 # cursor_factory
 # ==============