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https://docs.djangoproject.com/en/dev/topics/db/queries/#querysets-are-lazy
I've known this for quite some time, but never had the chance to really use this, until recently I was going over some old code. Someone has asked me to implement a search feature where objects are being searched on by some attributes. This was still when I was just starting out in python so the search was implemented poorly a lot of filtering was done in the app itself and filter building was really awkward and QuerySets were being evaluated for intermediate results. The logic basically consisted of a loop that would take a filter condition and create a query set, then combine resulting query sets together and return them.
This worked ok, until recently someone pointed out that the proper way would be to apply the AND condition to all the filters or at least provide an option to do so.
Knowing that QuerySets are lazy I knew it was safe to come up with something like this
reduce(lambda acc, x: acc.filter(criteria=x), xs, myobj.objects.all())
This would repeatedly apply the filter and also allow me to construct filters on the fly.
Oh but you might say what if you are not searching by field called criteria. Well you can actually define the field and the value you are searching for, however rather then me copying and pasting someone else's work, just read this for further explanation: http://www.nomadjourney.com/2009/04/dynamic-django-queries-with-kwargs/
My whole point is that you can apply successive filters without any time penalty. Not a single db query will be sent until you evaluate the query set.
