this raises:

Traceback (most recent call last):
  File "test.py", line 25, in <module>
    p.tags.add('a new tag')
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/ext/associationproxy.py", line 900, in add
    self.col.add(self._create(value))
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/ext/associationproxy.py", line 861, in _create
    return self.creator(value)
  File "/Users/classic/dev/sqlalchemy/lib/sqlalchemy/ext/associationproxy.py", line 492, in <lambda>
    creator = lambda *args: creator(self.lazy_collection.obj, *args)
AttributeError: '_lazy_collection' object has no attribute 'obj'