On 28.02.2006., at 17:37, Michael Bayer wrote:
how many classes do you want to be able to add to this ? doing a
giant SELECT that outerjoins many tables, with only one being used
for each row, will perform very poorly.
4-5 classes
Yes it would be slow, but I see two alternatives:
1) have a big table which combines all the objects (having a phisical
layout similar to the output of the outerjoin (without the foreign keys)
which is maybe faster
2) generate a new select for every child in order to get the
additional attributes (perhaps lazily)
One solution could be doing N selects one for each class and combine
the result but would work only for one level of inheritance.
For example I assume that there can be instances of Base that are not
Persons and Organizations. Combining Persons and Organizations would
exclude pure Bases. Including all Base objects will duplicate
objects. (this also apply for deeper hierarchies)
Do you mean slow because of sending null values accross the
connection or slow doing the select itself?
One possible solution would be to create a select that selects only
bases for which there are no related subclass instances but I imagine
the overhead would be very similar to the outerjoin in term of index
access (still faster because will not send null fields accross the
connection).
One of the reasions I need to do that is that with zope3 I can assign
particular views/viewlets/adapters with particular classes and
exploit inheritance.
anyway, create_instance is not too different from the append_result
you already figured out:
class MyExt(MapperExtension):
def create_instance(self, mapper, row, imap, class_):
if row['col1'] == 'foo':
return Organization(_mapper_nohistory=True)
else:
return Person(_mapper_nohistory=True)
Ah nice.
When I select objects from Person.select() I get the same instance.
Is possible to obtain this behaviour using custom object creation?
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