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Using Queries with Fetch Groups

from Using Advanced Query API (ELUG) - http://wiki.eclipse.org/Using_Advanced_Query_API_%28ELUG%29

You can use a fetch group with a ReadObjectQuery or ReadAllQuery. When you execute the query, EclipseLink retrieves only the attributes in the fetch group. EclipseLink automatically executes a query to fetch all the attributes excluded from this subset when and if you call a getter method on any one of the excluded attributes.

Note: When you use fetch groups outside of CMP, use weaving (see Using Weaving).

This section describes the following:

• How to Configure Default Fetch Group Behavior
• How to Query with a Static Fetch Group
• How to Query with a Dynamic Fetch Group

For more information about fetch groups, see Fetch Groups and Object-Level Read Queries.

How to Configure Default Fetch Group Behavior

You can optionally designate at most one fetch group as the default fetch group for a descriptor's reference class.

If you execute a ReadObjectQuery or ReadAllQuery without specifying a fetch group, EclipseLink will use the default fetch group unless you configure the query otherwise, as this example shows.

Configuring Default Fetch Group Behavior

// at the descriptor level
FetchGroup group = new FetchGroup("nameOnly");
group.addAttribute("firstName");
group.addAttribute("lastName");
employeeDescriptor.getFetchGroupManager().addFetchGroup(group);
// set the default fetch group
employeeDescriptor.getFetchGroupManager().setDefaultFetchGroup(group);

// when query1 is executed, the default fetch group applies
ReadAllQuery query1 = new ReadAllQuery(Employee.class);

// when query2 is executed, the default fetch group does not apply
ReadAllQuery query2 = new ReadAllQuery(Employee.class);
query2.setShouldUsedefaultFetchGroup(false);

How to Query with a Static Fetch Group

Configuring a Query with a FetchGroup Using the FetchGroupManager shows how to configure a ReadObjectQuery for the Employee class with a FetchGroup named nameOnly previously stored in the FetchGroupManager owned by the Employee class's descriptor.

' Configuring a Query with a FetchGroup Using the FetchGroupManager

In this example, only the Employee attributes firstName and lastName are fetched. If you call the Employee method get for any other attribute, EclipseLink executes another query to retrieve all unfetched attribute values. Thereafter, calling that get method will return the value directly from the object.

// create static fetch group at the descriptor level
FetchGroup group = new FetchGroup("nameOnly");
group.addAttribute("firstName");
group.addAttribute("lastName");
descriptor.getFetchGroupManager().addFetchGroup(group);

// use static fetch group at query level
ReadAllQuery query = new ReadAllQuery(Employee.class);
query.setFetchGroupName("nameOnly");

How to Query with a Dynamic Fetch Group

Configuring a Query with a FetchGroup Dynamically shows how to create a FetchGroup instance dynamically, at the time you create and execute a query, and configure the query with that FetchGroup directly.

In this example, only the firstName, lastName, and salary attributes are fetched. If you call the Employee method get for any other attribute, EclipseLink executes another query to retrieve all unfetched attribute values. Thereafter, calling that get method will return the value directly from the object.

Configuring a Query with a FetchGroup Dynamically

// dynamic fetch group query
ReadAllQuery query = new ReadAllQuery(Employee.class);
FetchGroup group = new FetchGroup("nameAndSalary");
group.addAttribute("firstName");
group.addAttribute("lastName");
group.addAttribute("salary");
query. setFetchGroup(group);