Django自定义Manager重命名字段后，get与filter为何表现不同？

Great question! Let's break down exactly what's happening here with your custom Manager and the key differences between how Django handles filter() and get() when working with aliased fields.

First, a quick recap of your setup: you’ve overridden get_queryset() to return a ValuesQuerySet that renames original_field to renamed_field using an F() expression.

The Root Cause #

The issue boils down to two things: how Django resolves aliased fields in query conditions and a fundamental SQL execution rule:

1. When you use Model.objects.filter(renamed_field='Test'):

   • Django recognizes that renamed_field is defined as a reference to original_field via the F() expression in your values() call. It smartly resolves the alias back to the actual database field, generating SQL that looks like this:

     SELECT original_field AS renamed_field FROM your_model WHERE original_field = 'Test';
     

   • This works because the filter condition targets the real database field (original_field), which is valid in the WHERE clause.

2. When you use Model.objects.get(renamed_field='Test'):

   • Django’s get() method doesn’t handle the alias resolution the same way. Instead, it tries to use renamed_field directly in the WHERE clause, producing SQL like this:

     SELECT original_field AS renamed_field FROM your_model WHERE renamed_field = 'Test';
     

   • SQL doesn’t allow using SELECT aliases in the WHERE clause (since WHERE runs before SELECT, the alias doesn’t exist yet). This query returns no matching rows, hence the DoesNotExist error.

Fixes to Try #

Here are a few straightforward solutions depending on your needs:

Option 1: Use the original field in get() (quick fix)

Since get() can’t resolve the alias, you can explicitly reference the original field name:

Model.objects.get(original_field='Test')

Option 2: Switch to annotate() instead of values() (cleanest long-term fix)

If you want the renamed field to work across all query methods (including get()), replace values() with annotate(). This adds renamed_field as an annotation to your model instances, so Django can properly resolve it in any query:

class CustomManager(models.Manager):
    def get_queryset(self):
        return super().get_queryset().annotate(renamed_field=F('original_field'))

Now both filter() and get() will work seamlessly with renamed_field.

Option 3: Manually resolve the alias in get()

If you need to stick with values(), you can use the F() expression directly in your get() condition to target the original field:

Model.objects.get(F('original_field')='Test')

内容的提问来源于stack exchange，提问作者Joshua