I know this should be pretty basic but somehow I don't quite get it. I want to get all users which age is lesser than 18 so the query should be something like this
User.objects.filter(age < 18)
what Im doing wrong?
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1 Answer
In order to filter with a less than filter, you use the __lt lookup [Django-doc]:
User.objects.filter(age__lt=18)or if you want to filter on a property that is some expression of fields, you can first annotate:
from django.db.models import F
User.objects.annotate(
age=F('age1') + F('age2')
).filter(age__lt=18)or if you want to subtract a number:
from django.db.models import F
User.objects.annotate(
ageminus=F('age') - 5
).filter(ageminus__lt=18)In this example the User object has no age field, but an age1 and age2 field. First we thus introduce an annotation age, that is the sum of these two fields.
By writing .filter(age < 18), the Python interpreter will look for a variable named age, and if that indeed exists (not per), then it will compare that with 18, and pass the result as a positional parameter to filter(..). So unless you use some proxy objects, like SqlAlchemy does, that will not work.
answered Dec 5, 2019 at 17:57
willeM_ Van Onsem
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5 Comments
Anibal Cardozo
I simplified the problem to make it easier to understand, but can I do opperations inside the filter? something like User((age__ - aux)lt=18) ? I need to know if the age minus X number is still < 18
willeM_ Van Onsem
@AnibalCardozo: then you usually make
.annotate(..)s and filter on these: docs.djangoproject.com/en/3.0/ref/models/querysets/#annotateAnibal Cardozo
I see what you mean, annotate seems to be the answer, but I still have a problem I get an error when using the attribute if put something like this User.objects.annotate(age - X < 18) I get an error with "name 'age' is not defined" If I try Users.objects.annotate ('age' - X < 18) I get an error like "unsupported operand type(s) for -: 'str' and 'int'"
willeM_ Van Onsem
@AnibalCardozo: but for the same reasons as listed in the answer for filtering, you can not do this for annotation. See the edited answer for some examples on how to annotate.
Anibal Cardozo
I figured it out a solution for my specific case, the answer was actually a lot simpler, instead of doing the operation inside the filter I could do it outside, something like this ``` real_age = X+18 User.objects.filter(age__lt=real_age) ```