Is there a way to annotate django query with non-expression?

Question

I have use case where I need to get all objects where existing_field is the beginning of some string.

some string changes dynamically so I need a smart way to filter out objects.

My idea is to create annotated query like this:

MyModel.objects.annotate(annotated_field='some string').filter(annotated_field__startswith=F('existing_field'))

Currently it is failing with: QuerySet.annotate() received non-expression(s): some string

Is there a way to annotate objects with string value?

score 16 · Accepted Answer · edited Oct 16 '19 at 11:59

16

Not sure what you're asking but try Value expression.

MyModel.objects.annotate(annotated_field=Value('some string', output_field=CharField())).filter(annotated_field__startswith=F('existing_field'))

edited Oct 16 '19 at 11:59

makozaki

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answered Oct 16 '19 at 10:25

kyore

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Great thanks. Solved it in the meantime like you said but first annotated then filtered. `annotated_field` needs to be created first. – makozaki Oct 16 '19 at 11:32

score 2 · Answer 2 · answered Feb 25 '22 at 00:13

For those who are still facing the problem with newer versions of Django will have to use ExpressionWrapper and probably F

from django.db.models import ExpressionWrapper, F
MyModel.objects.annotate(annotated_field=ExpressionWrapper(Value('some string', output_field=CharField()))).filter(annotated_field__startswith=F('existing_field'))

Is there a way to annotate django query with non-expression?

2 Answers2