Transaction to find an entity - locks all entities of that type?

Reading the docs for transactions:

http://code.google.com/appengine/docs/java/datastore/transactions.html

An example provided shows one way to make an instance of an object:

try {
    tx.begin();

    Key k = KeyFactory.createKey("SalesAccount", id);
    try {
        account = pm.getObjectById(Employee.class, k);
    } catch (JDOObjectNotFoundException e) {
        account = new SalesAccount();
        account.setId(id);
    }

    ...

When the above transaction gets executed, it will probably block all other write attempts on Account objects? I'm wondering because I'd like to have a user signup which checks for a username or ema开发者_JS百科il already in use:

tx.begin();

"select from User where mUsername == str1 LIMIT 1";
if (count > 0) {
    throw new Exception("username already in use!");
}

"select from User where mEmail == str1 LIMIT 1";
if (count > 0) {
    throw new Exception("email already in use!");
}

pm.makePersistent(user(username, email)); // ok.

tx.commit();

but the above would be even more time consuming I think, making an even worse bottleneck? Am I understanding what will happen correctly?

Thanks

---

No, transactions only operate on Entity Groups, that is, the set of entities with the same root entity. The grouping has nothing at all to do with entity Kind; an entity's parent can be of any type.

By default, all of your entities are root entities, which means that each is an entity group of 1 entity. Unless you explicitly set a parent entity when you create a new entity, this is the behavior you'll get.