When last_version is deleted, do not delete text and previous versions in cascade, but really delete text only when there is no previous version, otherwise update text.last_version with previous version.
from setuptools import setup, find_packages
setup(
name = "comt",
version = "2.0-alpha1",
url = 'http://www.co-ment.org',
license = 'AGPL3',
description = "Web-based Text Annotation Application.",
author = 'Sopinspace',
author_email = 'dev@sopinspace.com',
packages = find_packages('src'),
package_dir = {'': 'src'},
install_requires = ['setuptools'],
)