docs: def _encode_dbref(name, value, check_keys, opts):
      """Encode bson.dbref.DBRef."""
      buf = bytearray(b"\x03" + name + b"\x00\x00\x00\x00")
      begin = len(buf) - 4
  
      buf += _name_value_to_bson(b"$ref\x00",
                                 value.collection, check_keys, opts)
      buf += _name_value_to_bson(b"$id\x00",
                                 value.id, check_keys, opts)
      if value.database is not None:
          buf += _name_value_to_bson(
              b"$db\x00", value.database, check_keys, opts)
      for key, val in iteritems(value._DBRef__kwargs):
          buf += _element_to_bson(key, val, check_keys, opts)
  
      buf += b"\x00"
      buf[begin:begin + 4] = _PACK_INT(len(buf) - begin)
      return bytes(buf)