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having trouble understanding why I can't filter after a slice on a queryset and what is happening.
stuff = stuff.objects.all()
stuff.count()
= 7
If I then go
extra_stuff = stuff.filter(stuff_flag=id)
extra_stuff.count()
= 6. Everything is all good and I have my new queryset in extrastuff no issues
stuff = stuff.objects.all()[:3]
extra_stuff = stuff.filter(stuff_flag=id)
I get the error "Cannot filter a query once a slice has been taken."
How can I filter further on a queryset where I have limited the number of results?
796
The filter() method is used to filter you search, and allows you to return only the rows that matches the search term.
To do a not equal in Python Django queryset filtering, we can negate a equal with ~ . to call filter with the Q object negated with ~ to return all the Entry results that don't have id 3.
To filter a Python Django query with a list of values, we can use the filter method with in . to search Blog entries with pk set to 1,4 or 7 by calling Blog. objects. filter with the pk_in argument set to [1, 4, 7] .
Basically use get() when you want to get a single unique object, and filter() when you want to get all objects that match your lookup parameters.
You can't use filter() after you have sliced the queryset. The error is pretty explicit.
Cannot filter a query once a slice has been taken.
You could do the filter in Python
stuff = stuff.objects.all()[:3]
extra_stuff = [s for s in stuff if s.stuff_flag=='flag']
To get the number or items in extra_stuff, just use len()
extra_stuff_count = len(extra_stuff)
Doing the filtering in Python will work fine when the size of stuff is very small, as in this case. If you had a much larger slice, you could use a subquery, however this might have performance issues as well, you would have to test.
extra_stuff = Stuff.objects.filter(id__in=stuff, stuff_flag='flag')
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