keystone package needs sqlalchemy-migrate dependency

when installing keystone using the following ppa:

  deb http://ppa.launchpad.net/keystone-core/trunk/ubuntu natty main

and then using apt to install:

  apt-get install keystone

I can verify that the libraries are installed:

   python
  Python 2.7.1+ (r271:86832, Apr 11 2011, 18:13:53)
  [GCC 4.5.2] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>> import keystone.version
  >>> print keystone.version.version()
  2012.1-dev

but attempting to add a tenant leads to the following exception:

  # keystone-manage tenant add admin
  Traceback (most recent call last):
    File "/usr/bin/keystone-manage", line 13, in <module>
      import keystone.manage
    File "/usr/lib/python2.7/dist-packages/keystone/manage/__init__.py", line 31, in <module>
      from keystone.backends.sqlalchemy import migration
    File "/usr/lib/python2.7/dist-packages/keystone/backends/sqlalchemy/__init__.py", line 30, in <module>
      from migrate import exceptions as versioning_exceptions
  ImportError: No module named migrate

I was able to to get this command working by installing an additional package:

  apt-get install python-migrate