Django: Loop Queryset inside Queryset

Question

So I have queryset1 that filters ModelA by 2 parameters:

queryset1 = ModelA.objects.all().filter(par1="X",par2="Y")

Then I have another queryset2 that I would like to filter ModelB by grabbing the elements that have already been filtered by queryset1, and also that follow another parameter:

queryset2 = ModelB.objects.all().filter(par3="X" for i in queryset,par4="Z")

My question is: Is there a way to perform a queryset on a queryset that you have already made? In the end I would like to pull the objects from ModelA and ModelB that follow par1 par2 par3. How would I do this?

I don't understand what are you trying to do, can you show us your model? What's the relationship between ModelA and ModelB? What are you trying to do with for i in queryset? — Shang Wang
– Shang Wang, Commented Jul 29, 2016 at 20:15
ModelA and ModelB have some of the same objects, but some different objects. I want to grab the objects that are the same in both models, then do a filter on those objects. — Connor Meeks
– Connor Meeks, Commented Jul 29, 2016 at 20:18
Oh that's what you are doing? No, in django you can't filter on 2 models. Queryset needs to be objects from one model only. — Shang Wang
– Shang Wang, Commented Jul 29, 2016 at 20:29

e4c5 · Accepted Answer · 2016-07-30 00:26:31Z

1

it's quite possible to achieve without a for loop. The disadvantage of using a for loop is that you are actually executing two queries and retrieving the results for the first one. If that happens to produce a large number of rows you are burning up resources and slowing down the response time.

queryset2 = ModelB.objects.all().filter(par3__in=queryset,par4="Z")

This would result in a subquery as explained at https://docs.djangoproject.com/en/1.9/ref/models/querysets/

You can also use a queryset to dynamically evaluate the list of values instead of providing a list of literal values:

SELECT * FROM app_modelb where par3 IN (SELECT ...)

answered Jul 30, 2016 at 0:26

e4c5

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Todor · Accepted Answer · 2016-07-29 21:32:36Z

1

What you can do is:

When par3 is a relation of the same type of the queryset1:

queryset2 = ModelB.objects.all().filter(par3__in=queryset1, par4="Z")

Use the values_list('some_field', flat=True) notation, when part3 is not a relation

queryset2 = ModelB.objects.all().filter(par3__in=queryset1.values_list('X', flat=True), par4="Z")

edited Jul 29, 2016 at 21:32

answered Jul 29, 2016 at 20:51

Todor

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