6347 Cache the number of each component type assigned to devices/VMs

[arthanson]

Fixes: #6347

Adds counter fields to cache related model counts. Does this by adding a CachedCounterField type that hooks up post-save and post-delete signals on the related model, these hooked up by calling connect_counter in apps.py

also has change tracking, there is a TrackingModelMixin which I added to the base classes - by default it does nothing but if you set change_tracking_fields on the model to the fields to track it will do change tracking on them. The connect_counter method automatically does the correct fields (generally device_id) and then checks this on post_save signal to see if it has been moved and updates the counts on the old and new parent models appropriately.

The change tracking is probably useful elsewhere - you can check model.tracker.changed to see any changed fields. Note: these fields must be listed in change_tracking_fields.

[kkthxbye-code]

I know this is a draft, so apologies if this is premature. Looking at the code it doesn't look like it currently take into account that InventoryItems can be moved between devices.

It might also be an idea to add a management command to re-calculate counts, if they ever get out of sync.

[arthanson]

@kkthxbye-code could definitely use a more thorough review on this one, any feedback welcome.

[jeremystretch]

Excellent work! Just working on simplifying and cleaning things up a bit at this point. Also, could you add some simple tests for the counters? A single relationship (e.g. device interfaces) should be sufficient.

[arthanson]

@jeremystretch I added auto-discovery and cleaned up how you register / connect the fields, now you just supply it as params to the field:

    _interface_count = CounterCacheField(to_model='virtualization.VMInterface', to_field='virtual_machine')

and in apps.py ready function call:

        connect_counters(self)

I think this solves your issues with auto-discovery and the app registry? Please let me know if any additional changes.

It would be nice if there was a global app init, then could just run through all models and wouldn't have to remember to put connect_counters in each application apps.py.

Should I document using this in the models doc? Sort of an internal thing so not sure if this should go into the docs, but does effect people if they want to use this and add a field that should be cached.