On 24/04/2009, at 9:17 AM, Robert Zeigler wrote:
So, I was playing around a bit with the "is abstract" flag on
abstract entities.
I confess that it didn't quite work the way I thought it would.
For one, the superclass was abstract, but not the subclass.
That is the common situation when you want horizontal inheritance. [1]
I went ahead and made the subclass abstract. Which raises the
second issue. I have two obj entities entities extending the
"abstract" superclass, using a single discriminator column. I went
to query the base class (ala: select e from Entry e), and the query
failed, due to the inability to instantiate an instance of Entry.
Evidently, cayenne is trying to instantiate all of the subclasses
instances as instances of the superclass. Shouldn't cayenne be
instantiating the subclasses?
EOF has a switch for this called something like 'deep object
resolution'. That is, when you query Artist.class, should you get back
a list of Painter.class, Photographer.class, etc object (as you are
expecting), or get back a list of Artist objects. Each can be useful
in different situations, particularly since just returning Artist
objects means less work for the database (fewer JOINs).
Obviously if your query was for Painter.class then you will only get
Painter objects back.
Put another way, what is the technical reason that cayenne /isn't/
instantiating the subclasses as instances of the subclasses? Given
the potential of polymorphism, it seems like instantiating as the
superclass, even if the superclass is concrete, is incorrect
behavior? Just trying to clarify, here. I'll be digging through the
code to try to understand the specifics of how cayenne handles
inheritance, but, any pointers appreciated.
I think ultimately we need both behaviours.
Robert
Ari
[1] http://cayenne.apache.org/doc/inheritance-overview.html
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