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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