code: def _query(options, collection_name, num_to_skip,
             num_to_return, query, field_selector, opts, check_keys):
      
      encoded = _dict_to_bson(query, check_keys, opts)
      if field_selector:
          efs = _dict_to_bson(field_selector, False, opts)
      else:
          efs = b""
      max_bson_size = max(len(encoded), len(efs))
      return b"".join([
          _pack_int(options),
          _make_c_string(collection_name),
          _pack_int(num_to_skip),
          _pack_int(num_to_return),
          encoded,
          efs]), max_bson_size
docs: Get an OP_QUERY message.