Question

I have a rather large class stored in the datastore, for example a User class with lots of fields (I’m using java, omitting all decorations below example for clarity):

@PersistenceCapable
class User {

    private String username;
    private String city;
    private String state;
    private String country;
    private String favColor;
}

For some user queries, I only need the favColor property, but right now I’m doing this:

SELECT FROM " + User.class.getName() + " WHERE username == 'bob'

which should deserialize all of the entity properties. Is it possible to do something instead like:

SELECT username, favColor FROM " + User.class.getName() + " WHERE username == 'bob'

and then in this case, all of the returned User instances will only spend time deserializing the username and favColor properties, and not the city/state/country properties? If so, then I suppose all the other properties will be null (in the case of objects) or 0 for int/long/float?

Thank you

Answer 1

No, this isn’t possible with the App Engine datastore; your entire entity is stored in a protocol buffer and must be deserialized together.

If you have really large properties that aren’t often needed, it’s probably a good idea to put these in a separate model, although if you’re really talking about just 3 strings it’s almost certainly not worth the effort.