docs: 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])