def _get_reserved_field(field_id, reserved_fields):
  """Returns the desired reserved field.

  Args:
    field_id: str. The id of the field to retrieve.
    reserved_fields: list(fields). The reserved fields to search.

  Returns:
    The reserved field with the given `field_id`.

  Raises:
    ValueError: `field_id` is not a known reserved field.
  """
  matching_fields = [field for field in reserved_fields if field.id == field_id]
  if not matching_fields:
    raise ValueError('No reserved field with id `{}`'.format(field_id))
  return matching_fields[0]