Terrible performance on the county history diff page

[simonw]

Spotted this while working on #218:

We don't want to track the locations that belong to the county every time we edit the county - it will store a whole bunch of pointless data in our revision tables, plus it has a significant performance impact on the pages that show the differences between different county versions.

[simonw]

There's a follow= option in the registration API which should be able to fix this: https://django-reversion.readthedocs.io/en/latest/api.html#registration-api

[simonw]

Here's how to customize that when registering reversion using the VersionAdmin mixin: https://django-reversion.readthedocs.io/en/stable/admin.html#versionadmin-register

[simonw]

It looks like I misunderstood this: from digging into the database it doesn't look like those location lists are being included in the reversion serialized JSON records.

Instead this looks to be a performance issue with django-reversion-compare:

We can address this some point in the future.

[simonw]

Relevant issue: jedie/django-reversion-compare#95