Question

I am writing a web application (Tomcat 7) so no JTA and want to display a list of projects including the tasks associated to each project in a JSF page.

There are 2 entities – Project which contains a Collection of Tasks. From my Managed Bean I call a convenience method to retrieve all the active the Projects, then iterate (ui:repeat) over this Collection of Projects and want to display the Tasks per Project. When I try t do this I get a Persistence Exception since the Persistence Context is no longer in scope.

Without creating backing beans of the Project and Task and without making use of an Extended Context what other options do I have.

Answer 1

Have your convenience method also fetch the tasks, for instance by joining that association in the query? If the task are already loaded, JPA won’t need to fetch them when you access the collection.

Edit: The JPA 2.0 specification writes in section 4.4.5.3:

A FETCH JOIN enables the fetching of an association or element collection as a side effect of the execution of a query.

The syntax for a fetch join is

fetch_join ::= [ LEFT [OUTER] | INNER ] JOIN FETCH join_association_path_expression

The association referenced by the right side of the FETCH JOIN clause must be an association or element collection that is referenced from an entity or embeddable that is returned as a result of the query.

It is not permitted to specify an identification variable for the objects referenced by the right side of the FETCH JOIN clause, and hence references to the implicitly fetched entities or elements cannot appear elsewhere in the query.

The following query returns a set of departments. As a side effect, the associated employees for those departments are also retrieved, even though they are not part of the explicit query result. The initialization of the persistent state or relationship fields or properties of the objects that are retrieved as a result of a fetch join is determined by the metadata for that class — in this example, the Employee entity
class.

SELECT d 
FROM Department d LEFT JOIN FETCH d.employees
WHERE d.deptno = 1

A fetch join has the same join semantics as the corresponding inner or outer join, except that the related objects specified on the right-hand side of the join operation are not returned in the query result or otherwise referenced in the query. Hence, for example, if department 1 has five employees, the above query returns five references to the department 1 entity.

The FETCH JOIN construct must not be used in the FROM clause of a subquery.