Rachel Willmer wrote:
> I have a complicated query which should return the subset of a table
> whose objects match one of several conditions.
>
> What's the easiest way of doing this, given that I can't just do a SQL
> statement, because some of the conditions require me to feed the
> object's values into a function to evaluate it.
>
> I've had a few ideas:
>
> a) Pulling back the entire table, and deleting from the query set
> those objects which don't match.
>
> Is this possible? I get the feeling that using delete() would delete
> the entry from the db, which i don't want to do.
You shouldn't actually delete anything from a full list of records. Just
filter it in a new list (and then Python promptly discards unneeded
records from memory):
records = [r for r in Model.objects.all() if r.check_condition()]
(this assumes that check_condition returns something sensibly evaluating
to True or False)
> I can't find any documentation to show how to union 2 querysets - is
> it possible?
Since you anyway will get them in memory entirely you can just create
actual lists from querysets and then concatenate them (and may be check
for duplicates if needed):
result = list(queryset1) + list(queryset2)
But your option a) anyway looks more reasonable.
> c) Writing a custom Manager.
Managers don't do anything magical, you still have to choose wither a)
or b) way. Whether to place this code into a Manager or just let it lay
somewhere in the code is the whole different question, it's the matter
of convenience.
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