Fixes #19309: N+1 problem on /interfaces, /ip-addresses and /prefixes requests

[Tishka17]

Fixes: #19309

In case of requesting /interfaces, /ip-addresses and /prefixes some of the fields are selected separately for each model. We should prefetch them.

[jnovinger]

Hey @Tishka17 , there's one change I'd like to see for IPAddressViewSet. Other than that, this looks great. Sorry for the lengthy review time!

[jnovinger]

Sorry this took so long. I really wanted to understand why we needed to use the GenericPrefetch here, which meant some poking at things in my local.

So, it turns out that prefetch_related() can handle GFKs perfectly fine on its own, withouth the GenericPrefetch--which I'm guessing you already knew. I was pretty sure they did, but wanted to be sure I understood that correctly. As a matter of fact, prefetch_related() is also prefetching related VMInterface and FHRPGroups that are assigned via the GFK.

The interesting thing here, is what you did with the querysets argument, where you used select_related() on any associated interfaces to cut down any more incidental N+1 issues on that relation. So, I got to digging a bit and realized this is great and we can extend it a bit more, like so:

    queryset = IPAddress.objects.prefetch_related(
        GenericPrefetch(
            "assigned_object",
            [
                FHRPGroup.objects.all(),  # comment about why this is here
                Interface.objects.select_related("cable", "device"),
                VMInterface.objects.select_related("virtual_machine"),
            ],
        ),
    )

There are three changes here I'd like to see:

1. Add "cable" to the list of .select_related(...) arguments for Interface.
2. Add a VMInterface queryset with a .select_related("virtual_machine") since that also gets fetch if any IP Address is assigned to a VMInterface.
3. Add an FHRPGroup queryset added to the list for consistency. It's actually already picked up, but I think it makes sense to list it here since the others are as well so that future code readers aren't confused about why it's missing.