Linda,
If we decide that TREAT(x as T) returns NULL when the value x does not have type T, then does that produce portability issues, or can we suppose that it is always possible to map to appropriate DB syntax that will produce a NULL?
-----Original Message-----
From: Linda DeMichiel [mailto:linda.demichiel_at_oracle.com]
Sent: Monday, March 07, 2011 12:46 PM
To: jsr338-experts_at_jpa-spec.java.net
Subject: [jsr338-experts] query improvements: downcasting
As promised, we'll kickoff our work with some of the hopefully easier
items to get us warmed up. We'll start with some topics in the query
area.
First up -- downcasting.
We've gotten a number of requests to support a downcasting functionality
in the query language. This is useful particularly for polymorphic
relationships where the intent is to access subtype-specific attributes.
SQL supports this via the TREAT ... AS operator:
TREAT (expression AS datatype)
where datatype is a subtype of the static type of the expression.
The extension of this syntax to JPQL would be straightforward.
For example,
SELECT b.name, b.ISBN
FROM Order o JOIN TREAT(o.product AS Book) b
SELECT e FROM Employee e JOIN TREAT(e.projects AS LargeProject) lp
WHERE lp.budget > 1000
SELECT e FROM Employee JOIN e.projects p
WHERE TREAT(p AS LargeProject).budget > 1000 OR
TREAT(p AS SmallProject).name LIKE "Persist%" OR
p.description LIKE "COST OVERRUN"
An open issue however is the handling of the case where the
instance passed to TREAT is not of the same type or a subtype of the
specified datatype.
Databases seem to differ as to whether the result should be null or
whether an error is raised. We need to decide this as well.
I think the above examples illustrate the utility of defining the
semantics as providing a filtering as well as downcasting semantics
(i.e., taking the approach of using null semantics).
The Criteria API already provides the Expression method
<X> Expression<X> as(Class<X> type);
However, note that the definition of this explicitly states: "This
method does not cause type conversion: the runtime type is not
changed. Warning: may result in a runtime failure. "
Opinions?
thanks,
-Linda