On Apr 1, 2008, at 2:09 AM, pyplexed wrote:
>
>>
> Actually, I would know the type, so maybe that makes it simpler.
>
> I'm in a situation where I want to store data which is generated
> externally to my system, and which is polled regularly but individual
> items change relatively infrequently. It seemed sensible to try to
> quickly spot the items which are already in the database (i.e. all
> fields the same), as these require no further action.
>
> I would then process any of the remaining items which have changed
> singe the last data update, and create records for the new versions,
> and finally insert any completely new items into the database.
>
>>
It seems like you want to build a simple marshalling system which,
using a session, can convert data from your external system into a
mapped object graph. When doing this, you do the "query or create"
pattern you're hinting at, but thats about as hard as it gets. You
can fully repopulate all data each time without care for whether or
not its "changed" or not, the Session handles that all for you and
will only issue SQL for what's changed. I'd place the information
about how to "marshall" a particular class using classmethods, as below:
Session = scoped_session(....)
class MyClass(object):
@classmethod
def from_external_datasource(cls, arg1, arg2, arg3, arg4):
# locate existing record
obj = Session.query(cls).filter_by(arg1=arg1, arg2=arg2).first()
# create if not found
if not obj:
obj = MyClass(arg1, arg2)
Session.save(obj)
# populate "new" data
obj.arg3 = arg3
obj.arg4 = arg4
return obj
then you could loop through your external structure and build up the
collection using the load() method on each object. You dont need to
worry where the objects come from or if their data is the same or
different, that will all be sorted out when the session flushes. A
related collection, such as many-to-many, same idea:
@classmethod
def from_external_datasource(cls, arg1, arg2, arg3, arg4,
some_collection):
obj = <get as before>
obj.some_collection =
[RelatedClass.from_external_datasource(s.id)
for s in some_collection]
return obj
Note that you can entirely rebuild the collection from scratch - the
Session will still only issue INSERTs or DELETEs for the items which
are different.
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