Better to use Key or User in referencing Users?

[Carter]

Is a query on type User more expensive than querying on type Key?

In storing references to users of your application across multiple
Kinds, which is better the better approach assuming I don't need
locking for transactions?

a) store the "User" type (the actual User object) or
b) store a "Key" type (that was used as the Primary Key for a Kind
that stores user information)

For example, if the Kind AppUser is defined as...

@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class AppUser {
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key key;

@Persistent
private User user;
...
}

and we're storing an image uploaded by that AppUser, is it better to
use:

Option (a)

@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class UploadedImage {
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key key;

@Persistent
private User user;
...
}

or, Option (b)

@PersistenceCapable(identityType = IdentityType.APPLICATION)
public class UploadedImage {
@PrimaryKey
@Persistent(valueStrategy = IdGeneratorStrategy.IDENTITY)
private Key key;

@Persistent
private Key appUserKey;
...
}

When retrieving 100s of images owned by the currentUser(), is option
(b) preferred even though it would require the initial lookup of the
AppUser before querying UploadedImage using appUserKey?

[Nick Johnson (Google)]

Hi Carter,

Querying on one property or another has effectively no difference in performance. There are other considerations, however:

- Using a key will allow you to potentially add support for users authenticated in other ways (Eg, an external authentication framework) without changing the rest of your models.

- If you set the key_name of the userinfo model to the user_id of the user, you don't need to look the record up - you can create its key based on the logged in user with no datastore operations

- Querying on user objects can be tricky, particularly if the user has changed their email address. It's best to avoid doing this if you can.

-Nick Johnson

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