Using Queries with Fetch Groups

You can use a fetch group with a ReadObjectQuery or ReadAllQuery. When you execute the query, TopLink retrieves only the attributes in the fetch group. TopLink 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.

This section describes the following:

• Configuring Default Fetch Group Behavior

• Querying with a Static Fetch Group

• Querying with a Dynamic Fetch Group

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

Configuring 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, TopLink will use the default fetch group unless you configure the query otherwise, as Example 99-3 shows.

Example 99-3 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);

Querying with a Static Fetch Group

Example 99-4 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.

Example 99-4 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, TopLink 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");

Querying with a Dynamic Fetch Group

Example 99-5 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.

Example 99-5 Configuring a Query with a FetchGroup Dynamically

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

// 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);