如何在 Django ORM 中获取常量列？

Question

I am studying ORM. However, there are some things I don't understand even after reading the documentation.

How do I get a constant column in a Django ORM?

SQL is here.

select *, 'a' as a from book;

This is the query set I tried

Book.objects.annotate(a=Value('a'))

The error is here.

django.core.exceptions.FieldError: Cannot resolve expression type, unknown output_field

Answer 1

When executing your code, I get the following error:

FieldError('Cannot resolve expression type, unknown output_field')

Which means django doesn't know the type that your annotated field should result in.

To fix this, you need to wrap your annotation in an ExpressionWrapper :

from django.db.models import Value, ExpressionWrapper

Book.objects.annotate(a=ExpressionWrapper(
    Value('a'), 
    output_field=models.CharField(max_length=45),
))