def relationship_type(self, name):
try:<EOL><INDENT>cls = self._relationship_types[name]<EOL><DEDENT>except KeyError:<EOL><INDENT>cls = self._relationship_types[name] = type(str(name), (Relationship,), {})<EOL><DEDENT>return cls<EOL>
Obtain a :class:`.Relationship` subclass for a given
        relationship type name.