Allow queryset for RelatedField to be a callable

[tuky]

I would like to suggest this to make 2 use cases easier:

1. The callable could optionally receive the context so you can for example filter on the authenticated user without overriding each such field (see Document how to handle permissions and filtering for related fields. #1985)
2. We have querysets like Package.objects.filter(expiration__lt=timezone.now()) which we really need to be lazy.

What do you think? Thx!

[alextatarinov]

I agree this would be a useful feature.
I feel like a better option could be similar to custom field validation, but instead of def validate_field_name(self, value) it can be def get_queryset_field_name(self, field). Having access to self is quite useful and the following typical pattern definitely won't fit into lambda:

def get_fields(self):
    fields = super().get_fields()

    if getattr(self.context['view'], 'swagger_fake_view', False):
        queryset = Model.objects.none()
    else:
        user = self.context['request'].user
        queryset = Model.objects.filter(owner=user)

    fields['field_name'].queryset = queryset
    return fields

[tuky]

great idea. it would be also similar to how SerializerMethodField works. maybe we'd need a queryset_method_name parameter, too.