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I'm working on an app that requires me to filter through large amount of records. I've been reading about caching QuerySets and related stuff and found some good material. For example:

Caching query results in django https://docs.djangoproject.com/en/dev/topics/db/queries/#caching-and-querysets

also something about QuerySet working etc. https://docs.djangoproject.com/en/dev/topics/db/optimization/#understand-queryset-evaluation https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.iterator

There are some things still unclear though.

In my app it is somewhat like this: [I do have validations etc, not written here]

qs = MyModel.objects.filter(Q( <initial_filter_to_narrow_down_size>  ))

#I let user to specify other filters and I use qs further

q_object = Q(< [using user_made_filters].pop()> )
for filter in user_made_filters:
    q_object |= Q( <using filter> )

qs = qs.filter(q_object)

there can be n number of user_made_filters. For some of them I need to do &= instead of |=.

Questions:- 1]

qs = MyModel.objects.filter(Q(<initial_filter_to_narrow_down_size>))

After this, I wish to put this qs in cache for later use. I want to apply all the other filters without hitting the database. something like 

cache.set('qs', qs)

but what happens when I will do qs = qs.filter(q_object) ? Cache will be modified ? I don't want that. I want qs to remain constant until I update it. What should I do in this case ?

2] Like I said before, I generate OR'ed or ANDed Q objects and I keep doing qs = qs.filter(q_object). I do this because I find similar type of filters at once, apply them then get another type of filters and go on. I also need to prioritize some filters. They will be applied first and then rest of them. is there any difference between: asking this because in my idea my qs is of type (a).

(a) MyModel.objects.filter( Q(condition1) ).filter( Q(condition2) ) and
(b) MyModel.objects.filter( Q(condition1) & Q(condition2) ) ?

I mean when qs is finally evaluated, will django evaluate as per (a) ? or will it combine all filters and evaluate it like (b) ?

Regards,

ps- won't be able to reply back for 2-3 hours.

Community
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Omie
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1 Answers1

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1) No, the cache value won't be modified, unless you set it again.
2) Yes, applying two filters is like ANDing them. You can verify the two querysets produce the exact same sql by writing:

qs1 = MyModel.objects.filter( Q(condition1) ).filter( Q(condition2) )
qs2 = MyModel.objects.filter( Q(condition1) & Q(condition2) )
unicode(qs1.query) == unicode(qs2.query)
Germano
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