Fetch groups are a standard feature of JDO, but are not part of the JPA specification. This section describes Kodo's JPA fetch group implementation.

Kodo places any field that is eagerly loaded according to the JPA metadata rules into the built-in default fetch group. The default fetch group is always active; you cannot remove it at runtime. Thus fields in this group are always loaded immediately when an object is fetched from the datastore.

A field can be a member of zero or one fetch groups, including the default fetch group. That is, fields in the default fetch group cannot be in an additional fetch group, and a field cannot declare itself a member of more than one fetch group. So to place a field in a custom fetch group, you must first exclude it from eager fetching in your JPA metadata, if it does not already default to lazy loading.

When lazy-loading a field, Kodo checks to see if that field declares itself to be a member of a fetch group. If so, Kodo will load all fields in that fetch group.

Additionally, it is possible to configure a Kodo EntityManager or Query to use a particular fetch group or set of fetch groups when loading new objects, as described later in this chapter. When this is the case, Kodo loads the default fetch group plus any fields in the specified set of additional fetch groups.

You create fetch groups with the kodo.persistence.FetchGroup annotation. If your class only has one custom fetch group, you can place this annotation directly on the class declaration. Otherwise, use the kodo.persistence.FetchGroups annotation to declare an array of individual FetchGroup values. The FetchGroup annotation has the following properties:

As you might expect, listing a kodo.persistence.FetchAttribute within a FetchGroup includes the corresponding persistent field or property in the fetch group. Each FetchAttribute has the following properties:

Thus, to create a detail fetch group consisting of the publisher and articles relations, with the fetch group applied recursively to the related objects, use:

import kodo.persistence.*;

@Entity
@FetchGroups({
    @FetchGroup(name="detail", attributes={
        @FetchAttribute(name="publisher" depth=0),
        @FetchAttribute(name="articles" depth=0)
    }),
    ...
})
public class Magazine
{
   ...
}

	Note
	Kodo currently only supports a depth of 0 for fetch attributes. This restriction will be lifted in a future release, along with the restriction limiting each attribute to a single fetch group.

You can control the default set of fetch groups with the kodo.FetchGroups configuration property. Set this property to a comma-separated list of fetch group names.

In JPA, Kodo's KodoEntityManager and KodoQuery extensions to the standard EntityManager and Query interfaces provide access to a kodo.persistence.FetchPlan object. The FetchPlan maintains the set of active fetch groups. It begins with the groups defined in the kodo.FetchGroups property, but allows you to add and remove groups for an individual EntityManager or Query through the methods below.

public FetchPlan addFetchGroup (String group);
public FetchPlan addFetchGroups (String... groups);
public FetchPlan addFetchGroups (Collection groups);
public FetchPlan removeFetchGroup (String group);
public FetchPlan removeFetchGroups (String... groups);
public FetchPlan removeFetchGroups (Collection groups);
public FetchPlan resetFetchGroups ();
public Collection<String> getFetchGroups (); 
public void clearFetchGroups ();

Chapter 9, Runtime Extensions details the KodoEntityManager, KodoQuery, and FetchPlan interfaces.

import kodo.persistence.*;

...

KodoQuery kq = KodoPersistence.cast (em.createQuery (...));
kq.getFetchPlan ().addFetchGroup ("detail");
List results = kq.getResultList ();

In JDO, you can override the global FetchGroups property at runtime using the FetchPlan of individual PersistenceManagers, Queries, and Extents. See Chapter 12, FetchPlan for details. The extended KodoFetchPlan interface adds an additional useful methods for manipulating fetch groups:

public void resetGroups ();

resetGroups resets the active fetch group set to the groups defined in the kodo.FetchGroups configuration property. Chapter 9, Runtime Extensions provides more information on Kodo extensions to standard JDO interfaces.

In addition to controlling fetch configuration on a per-fetch-group basis, you can configure Kodo to include particular fields in the current fetch configuration. This allows you to add individual fields that are not in the default fetch group or in any other currently-active fetch groups to the set of fields that will be eagerly loaded from the database.

JPA FetchPlan methods:

public FetchPlan addField (String field);
public FetchPlan addFields (String... fields);
public FetchPlan addFields (Class cls, String... fields);
public FetchPlan addFields (Collection fields);
public FetchPlan addFields (Class cls, Collection fields);
public FetchPlan removeField (String field);
public FetchPlan removeFields (String... fields);
public FetchPlan removeFields (Class cls, String... fields);
public FetchPlan removeFields (Collection fields);
public FetchPlan removeFields (Class cls, Collection fields);
public Collection<String> getFields (); 
public void clearFields ();

JDO KodoFetchPlan methods:

public KodoFetchPlan setFields (String[] fields);
public KodoFetchPlan setFields (Class cls, String[] fields);
public KodoFetchPlan setFields (Collection fields);
public KodoFetchPlan setFields (Class cls, Collection fields);
public KodoFetchPlan addField (String field);
public KodoFetchPlan addField (Class cls, String field);
public KodoFetchPlan removeField (String field);
public KodoFetchPlan removeField (Class cls, String field);
public boolean hasField (Class cls, String field);
public Collection getFields (); 
public KodoFetchPlan clearFields ();

The methods that take only string arguments use the fully-qualified field name, such as org.mag.Magazine.publisher. Similarly, getFields returns the set of fully-qualified field names. In all methods, the named field must be defined in the class specified in the invocation, not a superclass. So, if the field publisher is defined in base class Publication rather than subclass Magazine, you must invoke addField (Publication.class, "publisher") and not addField (Magazine.class, "publisher"). This is stricter than Java's default field-masking algorithms, which would allow the latter method behavior if Magazine did not also define a field called publisher.

In order to avoid the cost of reflection, Kodo does not perform any validation of the field name / class name pairs that you put into the fetch configuration. If you specify non-existent class / field pairs, nothing adverse will happen, but you will receive no notification of the fact that the specified configuration is not being used.

import kodo.persistence.*;

...

KodoEntityManager kem = KodoPersistence.cast (em);
kem.getFetchPlan ().addField (Magazine.class, "publisher");
Magazine mag = em.find (Magazine.class, magId);

import kodo.jdo.*;

...

PersistenceManager pm = ...;
KodoFetchPlan fetch = (KodoFetchPlan) pm.getFetchPlan ();
fetch.addField (Magazine.class, "publisher");
Magazine mag = (Magazine) pm.getObjectById (magId);