Prefetch invalidation on an update operation

[Clément Hallet]

Hello,

In a ModelViewset, a prefetch_related is set with an extra "order_by" :

def filter_queryset(self, qs):
    qs = super().filter_queryset(qs)
    qs = qs.prefetch_related(
        Prefetch(
            "related_data",
            queryset=Record.objects.order_by(
                "ts_update"
            ),
        ),
    )
    return qs

It is well applied on DRF retrieve operation. But on an update, after the changes have been applied, this prefetch_related is discarded. I completly understand why the cached data must be refreshed after the perform_update. But in the case here, it also removes the semantic meaning of the query, leading to the resulting data differs between a PUT and a later GET calls.

I cannot act in the perform_update method since the invalidation comes after and the only workaround I can think of is :

def update(self, request, *args, **kwargs):
        super().update(request, args, kwargs)
        return self.retrieve(request)

But that doesn't feel right, replacing the response of a base method by the one from an other. Could it be done differently ?

Cheers,

--

Clément Hallet

[Linovia]

Indeed the preftech cache is emptied before returning the result of an update:

https://github.com/encode/django-rest-framework/blob/master/rest_framework/mixins.py#L73

This is due to some side effects with many to many fields after an update:

https://github.com/encode/django-rest-framework/issues/2442

I’m afraid you’ll have to override the update method.

Regards,

Xavier O.

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