[Django]-Django queryset union not working with only

by

2👍

However, this is also selecting only specified fields but why should I define fields 2 times?

Because the number of columns and their types of both SELECT statements should match, as is specified in the documentation on .union(…):

Passing different models works as long as the SELECT list is the same in all QuerySets (at least the types, the names don’t matter as long as the types are in the same order). In such cases, you must use the column names from the first QuerySet in QuerySet methods applied to the resulting QuerySet.

That being said, please don’t use .union(…) in the first place for this. You can merge the two conditions by wrapping these in Q objects and combining these with an "or":

from django.db.models import Q

doctors_models.Physician.objects.filter(
    Q(appointments__member=self.context['member']) |
    Q(appointments__booked_by=self.context['member'])
).only('id', 'name_prefix', 'first_name', 'middle_name', 'last_name', 'name_suffix')

Furthermore using .only(…) [Django-doc] or .defer(…) [Django-doc] is only advisable for columns that contain a large amount of data. For "small" columns, it will often not have a significant impact, and in case you later need these fields, it can even result in N+1 problems.

👤willeM_ Van Onsem

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