[sqlalchemy] Re: Multiple Levels of Inheritance

[Michael Bayer]

On Apr 3, 11:24 am, Chris Guin [EMAIL PROTECTED] wrote:
 Actually, it turns out I'm still getting NULL in the discriminator
 field of my base classes.  In my setup, an AspectDetection inherits
 from Detection which inherits from Event - the discriminator field of
 Detection is set properly, but is universally NULL in Event.  Any
 idea why this might be the case?



the same rules apply.  the discriminator column is physically present
on the event table only, and the event table must be present in
all queries for any of the subclasses.  Turning on SQL echoing
(especially with echo='debug' to show result sets) should readily show
why this column is coming out as NULL in some cases.
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[sqlalchemy] Re: Multiple Levels of Inheritance

[Chris Guin]

Thanks!  That appears to have done it.

Chris Guin

At 05:28 PM 3/27/2008, you wrote:


On Mar 27, 2008, at 3:54 PM, Chris Guin wrote:

 
  Does anyone know where I could find a working example of multiple
  levels of inheritance using joined table inheritance?
 
  Right now I have the following class hierarchy - an AspectDetection
  and an RFDetection are subclasses of Detection, which in turn is a
  subclass of Event.  Each of the classes has its own DB table, and an
  object can be simply an Event or a Detection.  I've tried mapping
  these classes together using the following code:
 
  detection_join =
  detection.outerjoin(aspect_detection).outerjoin(rf_detection)
  event_join = detection_join.outerjoin(event)

if the ultimate base class is Event, then the event table's columns
must be present in every result set.  By outerjoining (where outerjoin
is a LEFT OUTER JOIN) detection to event, you dont get Event objects
that are not Detection objects.  So event join should be:

 event_join=event.outerjoin(detection_join)

Similarly, mapping select_table directly to detection_join for
detection_mapper does not include any columns from the Event table, so
those loads would be failing pretty badly.   So for that mapper,
assuming you want all subclasses in one big query, youd want to set
select_table to:


event
.outerjoin
(detection).outerjoin(aspect_detection).outerjoin(rf_detection)

the good news is, 0.4.5 will deprecate select_table and you'll just be
able to say with_polymorphic='*' on all your mappers where you want
a join of all subtables constructed by default - it will do all this
work for you.





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