On Fri, Jul 25, 2014 at 11:36 PM, Marc Kirkwood <
marc.kirkw...@talktopebble.co.uk> wrote:
> When filtering a queryset with a *ValuesListQuerySet* instance in Django
> 1.6, different objects to that shown in the apparent list seem to be
> returned by the iterator.
>
> An illustration of a head-scratching debug session is shown below:
>
> (Pdb) latest
>
> *[4, 1]*
>
> (Pdb) keys.filter(pk__in=latest).values_list('pk')
>
> *[(3,), (1),)]*
>
> (Pdb) keys.filter(pk__in=[4, 1]).values_list('pk')
> *[(4,), (1),)]*
>
> Can anyone explain this behaviour to me? When I fully convert it with
> *list(latest)*, normal service resumes.
> I have a suspicion that it's because of the Postgres-aware *order_by()*
> and *distinct()* chain, in the queryset that *latest* was produced from.
>
It's difficult to say for certain without knowing details of all the moving
parts, but I'm going to guess that the query that is producing latest isn't
as consistent as you think it is.
In your debug session, statement 2 isn't "retrieve results that match
statement 1", it's "retrieve results that match this subquery". That is,
"latest" is being re-evaluated as a subquery in the second statement. If
there's any order sensitivity in the query generating "latest", you're not
guaranteed to get the same results.
However, if you call list(latest), it ceases to be a subquery - it takes
the cached results from the first statement and turns it into an IN
[values] clause - effectively the same as statement 3.
If there are lots of order_by and distinct clauses, that might also affect
exactly how the subquery is rolling out; it would be worth
printing keys.filter(pk__in=latest).query to see exactly what is being
executed.
Yours,
Russ Magee %-)
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