Re: Combining Querysets from different, but similar models

On Wed, Oct 5, 2011 at 8:37 AM, Steven Smith <steven@stevenlsmith.com> wrote:
> What's the cheapest way to do my queries and mix the results into one
> feed?

I'd say to do it in Python.

a simpler case: you have two querysets and want a single 'feed' with
results of one query followed by the other:

from itertools import chain

reviews = Review.objects.filter(....)
postings = MarketplacePosting.objects.filter(...)

combined_feed = chain(reviews, postings)

simple, eh? now a nicest addition would be to define an ordering for
each queryset on a similarly-named field and have the resultant
combination merges so the order is respected; something like this
(untested!):

def merge (fld, seqA, seqB):
iterA, iterB = iter(seqA), iter(seqB)
a, b = next(iterA), next(iterB)
while True:
if getattr(a,fld) <= getattr(b,fld):
yield a
try:
a = next(iterA)
except StopIteration:
while True:
yield next(iterB)
else:
yield b
b = next(iterB)
try:
b = next(iterB)
except StopIteration:
while True:
yield next(iterA)

--
Javier

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