Beta Draft: 2013-04-02
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
In JPQL, downcasting is accomplished in the
FROM clause, using
AS in the
Starting with JPA 2.0, 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.
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.
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
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
ExpressionBuilder). The parent expression of a cast must be an
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
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