About Query Casting

Use query casting to query across attributes in subclasses when using JPA or ORM. This feature is available in JPQL, EclipseLink Expressions, and Criteria API.

Starting with JPA 2.0, it is possible to limit the results or a query to those of a specific subclass. For example, the expression framework provides Expression.type(Class).

In JPQL, downcasting is accomplished in the FROM clause, using TREAT...AS in the JOIN clause.

The JPA Criteria API includes the casting operator Expression.as(type). This expression does a simple cast that allows matching of types within the generics.

Calling a cast on a JOIN node permanently alters that node. For example, in the example above, after calling join.as(LargeProject.class), the join refers to a LargeProject.

EclipseLink Expression Support for Downcasting

EclipseLink extends the Criteria API to allow a cast using Expression.as(type). The as method checks the hierarchy; and if type is a subclass of the type for the expression that is being called on, a cast is implemented.

The Expression.as(Class) can also be used for downcasting. The behavior of using Expression.as(Class) is as follows:

An exception is thrown at query execution time if the class that is cast to is not a subclass of the class of the query key being cast.
Casts are only allowed on ObjectExpressions (QueryKeyExpression and ExpressionBuilder). The parent expression of a cast must be an ObjectExpression.
Casts use the same outer join settings as the ObjectExpression they modify.
Casts modify their parent expression. As a result, when using a cast with a parallel expression, you must use a new instance of the parent expression.
Casting is not supported for TablePerClass inheritance.
It is prudent to do a check for type in a query that does a cast.
EclipseLink automatically appends type information for cases where the cast results in a single type; but for classes in the middle of a hierarchy, no type information is appended to the SQL.