Can you fetch me this thingie? Just for me please...

Kodo is a JDO implementation that it's not widely adopted and a bit more complex than Hibernate. Neverteless I found at least feature that it already had long before Hibernate did, called 'fetch groups'. With a fetch group you can for instance eager fetch a collection that is defaulty mapped as lazy for a certain scenario.

Let's say a bunch of addresses are defaulty mapped 'lazy' to a customer. We can then define a fetch group (eg 'addressFetchGroup') to indicate that this collection should be eagerly fetched when this group id is used in the query.

Here's what this would look like:

The package.jdo (mapping) file:

    <class name="Customer">
      <extension vendor-name="kodo" key="data-cache" value="false" />
      <extension vendor-name="kodo" key="jdbc-sequence-factory" value="native" />
      <extension vendor-name="kodo" key="jdbc-sequence-name" value="MYSEQUENCE" />
      <field name="name" persistence-modifier="persistent" />

      <field name="addresses" default-fetch-group="false">
        <collection element-type="address" />
        <extension vendor-name="kodo" key="fetch-group" value="addressFetchGroup"/>
      </field>

 ...

The code:

    public Customer lookUpCustomerWithAddresses(String customerId) {

        Collection fetchGroups = new ArrayList();
        fetchGroups.add("addressFetchGroup");
        fetchGroups.add("otherFetchGroup");
        ...
        return retrieveWithFetchGroups(fetchGroups, customerId);
    }

    @SuppressWarnings("unchecked")
    public <S> Collection<S> retrieveWithFetchGroups(Collection<String> fetchGroups, Object... parameters) {
    KodoQuery query = (KodoQuery) persistenceManager.newJDOQuery();
        try {
            query.getFetchConfiguration().addFetchGroups(fetchGroups);
            return (Collection) query.executeWithArray(parameters);
        } catch (Exception ex) {
  ...
        }
    }

When used correctly (and more importantly, measured correctly with a profiler), this can greatly improve performance of a specific use case, without affecting other use cases. As of hibernate 3.5 this feature is also included and known as 'fetch profiles'.