[rset] New method: ResultSet.one()
This method will return exactly one entity while enforcing its existence and unicity.
The idea is shamelessly borowed from SQLAlchemy Query.one().
Closes #3352314
[jcr: use len(self) instead of len(self.rows)]
# -*- coding: utf-8 -*-
# copyright %(year)s %(author)s, all rights reserved.
# contact %(author-web-site)s -- mailto:%(author-email)s
#
%(long-license)s
"""%(distname)s precreate script, executed at instance creation time or when
the cube is added to an existing instance, before the schema is serialized.
This is typically to create groups referenced by the cube'schema.
"""