def get_choices(self, entity=None):
return self.choices.all()<EOL>
Returns a list of name/title tuples::

    schema.get_choices()    # --> [("green", "Green color"), ("red", "Red color")]

Names are used for lookups, titles are displayed to user.

This method must be overloaded by subclasses of BaseSchema to enable
many-to-one schemata machinery.