Returns field's value prepared for interacting with the database
        backend.

        Used by the default implementations of ``get_db_prep_save``and
        `get_db_prep_lookup```

def get_db_prep_value(self, value, connection=None, prepared=False):
        """Returns field's value prepared for interacting with the database
        backend.

        Used by the default implementations of ``get_db_prep_save``and
        `get_db_prep_lookup```
        """
        if not value:
            return
        if prepared:
            return value
        else:
            assert(isinstance(value, list) or isinstance(value, tuple))
            return self.separator.join([unicode(s) for s in value])