捕获的异常：关键路径元素名称“ __xyz__”无效

Question

Using the PHP-GDS library, I receive an exception when trying to 'upsert' an Entity when the key is in the format "__xyz__".

Is there a way to work around this, as the key is a username which I may have no control over, and __xyz__ is a perfectly valid username. Here's the code and the exception message:

$username  =  "__xyz__";

$obj_user = new Entity();
$obj_user->setKeyName($username);
try {
  $result = $obj_user_store->upsert($obj_user);
}catch (Exception $e) {
   echo 'Caught exception: ',  $e->getMessage(), "\n";
   return false;
}

Exception:

Caught exception: The key path element name "__xyz__" is reserved.

Answer 1

Unfortunately no, you have to encode the username is some way. Or, I would recommend to use auto-generated keys, and store the user-provided data in values, not keys.

Documentation is pretty bad for this case, there's only one mention about that, in docs for superseded Python API for Datastore: https://cloud.google.com/appengine/docs/python/datastore/entities : "Key names cannot begin and end with two underscores __*__ ". Java documentation doesn't say no word about that. Nor modern Python API. And I couldn't find any PHP documentation for Datastore.

But it is the same for all APIs, because exception happens on db-side, so it doesn't matter which language you use (it's all protobufs down there).

I know only one usage of double underscore: when you build a query, and want to sort ascending/descending by keys. You can do that by sorting by non-existing reserved key named __key__ .