Group items tagged

Well, in AppEngine your primary key is the key for the object(Entity) and it can either be a generated ID or a unique string your application provides.  "Google App Engine" by Dan Sanderson has a lot of examples on this.  If you don't use a key_name property, then when you save with a put(), an ID is generated, but if you pass in key_name='thisIsMyKey' into the constructor, then you manually set the key for the object.  You can use the method, id_or_name() to return either the object's key name or its ID, which ever one it has and has_id_or_name() would return a boolean about it, and if it's not saved and your not using key_name, then the ID would not exists yet.  Also you can get the Entity(Object) from the datastore by using the get(k) method where k is a key object and the key object has two parts: kind and ID or key_name.  Additionally you can fetch an object from the datastore with get_by_id() and get_by_key_name()

The best option depends on your requirements. Here's a few solutions (I'm assuming you're using Python, since you didn't specify): If you need to do transactional updates on an entire tree, and you're not going to have more than about 1QPS of sustained updates to any one tree, you can use the built in support for heirarchial storage. When creating an entity, you can pass the "parent" attribute to specify a parent entity or key, and when querying, you can use the .ancestor() method (or 'ANCESTOR IS' in GQL to retrieve all descendants of a given entity. If you don't need transactional updates, you can replicate the functionality of entity groups without the contention issues (and transaction safety): Add a db.ListProperty(db.Key) to your model called 'ancestors', and populate it with the list of ancestors of the object you're inserting. Then you can easily retrieve everything that's descended from a given ancestor with MyModel.all().filter('ancestors =', parent_key). If you don't need transactions, and you only care about retrieving the direct children of an entity (not all descendants), use the approach outlined above, but instead of a ListProperty just use a ReferenceProperty to the parent entity. This is known as an Adjacency List. There are other approaches available, but those three should cover the most common cases.

   private LocalServiceTestHelper helper =          new LocalServiceTestHelper(new   LocalDatastoreServiceTestConfig().setBackingStoreLocation(DS_PATH)); You could copy the WEB-INF/appegine-generated/local_db.bin file with   your data next to your test case and in a @Before method, delete the   old copy and copy a new one to DS_PATH.  You might also want to check   out LocalDatastoreServiceTestConfig().setNoStrage(true)

I have a class which can contain many small elements in a list. Looks like: public class Farm {    private ArrayList<Horse> mHorses;} just wondering what will happen if the mHorses array grew to something crazy like 15,000 elements. I'm assuming that trying to write and read this from the datastore would be crazy, because I'd get killed on the serialization process. It's important that I can get the entire array in one shot without paging, and each Horse element may only have two string properties in it, so they are pretty lightweight: public class Horse {    private String mId;    private String mName;} I don't need these horses indexed at all. Does it sound reasonable to just store the mHorse array as a raw Text field, and force my clients to do the deserialization? Something like: public class Farm {    private Text mHorsesSerialized;} then whenever the client receives a Farm instance, it has to take the raw string of horses, and split it in order to reinstantiate the list, something like: // GWT client perhapsFarm farm = rpcCall.getMyFarm();String horsesSerialized = farm.getHorses();String[] horseBlocks = horsesSerialized.split(",");for (int i = 0; i < horseBlocks.length; i++) {    // .. continue deserializing the individual objects ...} yeah...

The GAE docs talk about how to setup a test to use the datastore.  But also follow the instructions here until the docs are fully updated.

The data loader feature communicates with your application running on App Engine using remote_api, a request handler included with the App Engine runtime environment that allows remote applications with the proper credentials to access the datastore remotely. To use remote_api, you must map a URL to it.