Hi,
I have a User class in my project, and I can't find a transactionally
safe method of creating relationships between users, because User
objects are always in their own entity group.
The simplest example I can come up with is a follow relationship (like
twitter). When a user follows another, both User objects should be
made aware of the relationship:
class User {
int mNumFollowing;
int mNumFollowers;
}
class Follow {
String mUsernameFollowed;
String mUsernameFollower;
}
Since both User objects involved are root entities, I can't use a
transaction to safely build the relationship. Ideally:
tx.begin();
Follow follow = new Follow(
userJoe.getUsername(),
userJane.getUsername());
pm.makePersistent(follow);
userJoe.mNumFollowing++;
pm.makePersistent(userJoe);
userJane.mNumFollowers++;
pm.makePersistent(userJane);
tx.commit();
I can make the Follow class an entity child of User, still the
relationship can't be done in a single transaction:
tx.begin();
userJoe.mNumFollowing++;
pm.makePersistent(followForJoe);
pm.makePersistent(userJoe);
tx.commit();
tx.begin();
userJane.mNumFollowing++;
pm.makePersistent(followForJane);
pm.makePersistent(userJane);
tx.commit();
the first transaction could succeed, and the second fail, leaving me
with a half-completed follow relationship.
What's the best way around same-entity-group-for-transactions in a
situation like this? Do we just have to deal with the fact that our
data might be incorrect? I could make User objects all children of a
single entity group, but that would probably be awful for performance
as every User object in the datastore would have to be locked to do a
transaction (just guessing, I don't know if that's true).
Thanks
--
You received this message because you are subscribed to the Google Groups
"Google App Engine" group.
To post to this group, send email to google-appeng...@googlegroups.com.
To unsubscribe from this group, send email to
google-appengine+unsubscr...@googlegroups.com.
For more options, visit this group at
http://groups.google.com/group/google-appengine?hl=en.
