Hello.
Thank you for the tips. I stick to my simple approach for the time being:
_rows = q.all()
rows = unwrap(_rows)
I need the cache within the context of one method only.
Ladislav Lenart
Od: "Michael Bayer" <[email protected]>
> On Sep 27, 2012, at 4:37 PM, Ladislav Lenart wrote:
>
> > Hello.
> >
> > Thank you for the explanation. I was relatively close :-)
> >
> > I understand the rationale but don't like the impact it has:
> > If I query the database for something, I expect it to be available via slot
> > access afterwards, but unless I hold onto the results, it won't be.
> >
> > I guess this is one of those tricky bits one has to be caution about :-)
>
> typically when I really want things to be "cached", I do this:
>
> cache = dict((obj.id, obj) for obj in session.query(Class))
>
> with those strong references set up, I control the scope of objects as though
> the Session weren't really involved.
>
> The Session has an option for a "strong referencing" identity map, which is a
> holdover from the old days when the whole "weak" idea wasn't available yet.
> I've tried to deprecate this approach though there are some users still using
> it. It should be in the docs if you look for it.
>
>
>
> >
> > Ladislav Lenart
> >
> >
> > On 27.9.2012 22:20, Michael Bayer wrote:
> >>
> >> On Sep 27, 2012, at 3:17 PM, Ladislav Lenart wrote:
> >>
> >>> Update.
> >>>
> >>> Te unwrap version works only if the original result of q.all() is kept
> >>> around:
> >>>
> >>> def test_access(self):
> >>> q = self.session.query(Foo, Bar).with_labels()
> >>> q = q.filter(Foo.bar_id == Bar.id)
> >>> # Only this combination works:
> >>> _rows = q.all()
> >>> rows = unwrap(_rows)
> >>> assert len(rows) > 0
> >>> with self.assert_no_sql_while():
> >>> for each in rows:
> >>> each.bar.data
> >>>
> >>> I have also rewritten unwrap to:
> >>>
> >>> def unwrap(items):
> >>> def f(each):
> >>> if isinstance(each, tuple) and len(each) > 0:
> >>> return each[0]
> >>> else:
> >>> return each
> >>> return map(f, items)
> >>>
> >>> because it is now completely useless as a generator.
> >>>
> >>> I am completely lost now. Does this have to do something with weak
> >>> references
> >>> and too eager garbage collection? I would expect that if a query fetches
> >>> some
> >>> objects, session will keep them all until explicitly removed from it to
> >>> eliminate successive SQL queries.
> >>
> >> the Session does not strongly reference items that have no pending changes
> >> on them. This allows code such as this:
> >>
> >> for criterion in crit:
> >> objects = session.query(Cls).filter(criterion).all()
> >> _process_objects(objects)
> >>
> >> above, if the Session strongly referenced all objects until explicitly
> >> removed, logic like the above would cause memory to grow unbounded.
> >>
> >> Of course, if the objects were referred to by other objects, such as
> >> members of a collection or inter-object reference, they get a strong
> >> reference via that collection, but in your example there is no linkage
> >> between "Foo" and "Bar" until you actually invoke the path between two
> >> particular instances.
> >>
> >> cPython uses referencing counting for garbage collection, so assuming an
> >> object has no reference cycles, it is garbage collected immediately as
> >> soon as its strong reference count goes to zero.
> >
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