docs: def get_true_false_both(query_params, field_name, default):
      '''Tries to get and return a valid of true, false, or both from the field
      name in the query string, raises a ValidationError for invalid values.'''
      valid = ('true', 'false', 'both')
      value = query_params.get(field_name, default).lower()
      if value in valid:
          return value
      v = ', '.join(sorted(valid))
      raise serializers.ValidationError({
          field_name: ['Must be one of [%s]' % v],
      })