Google App Engine通过键获取对象

Question

I am stuck with geting a persisted object by its id. I am getting an error:

com.google.appengine.api.datastore.EntityNotFoundException: No entity was found matching the key: CNG("T78")

I persist the object as below to the data store:

Key cngKey = KeyFactory.createKey("CNG", jsonCNG.cNGID);
Entity cngEntity = new Entity("CNG", cngKey);
cngEntity.setProperty("cng_name", jsonCNG.cNGName);
cngEntity.setProperty("cng_type", jsonCNG.cNGType);
cngEntity.setProperty("cng_content", cng);

Here cng is a json string. I set the key with a string: cNGID. I am trying to use the same id to get the object.

Key cngKey = KeyFactory.createKey("CNG", "T78")

and end up getting the above error.

Answer 1

您似乎尚未将其保存到数据存储中。

Datastore.put(cngEntity);

Answer 2

The constructor new Entity("CNG", cngKey) is defining the entity with kind and parent key . Then when you try to retrieve it you do not provide parent key: KeyFactory.createKey("CNG", "T78") . This does not work - you must either provide parent key in both places on not.

Note - defining entity parent is used when defining entity groups, which are in turn important when using transactions. You probably did not want that?

Instead you should just use new Entity(cngKey) .