Add queryset cache

[bayangan1991]

Add an optional cache to the resolution of extra fields and display conditions.

[Sticky-Bits]

Wait, so if we're not using the cache there's no relevant fields? That doesn't seem right?

[bayangan1991]

If we aren't using the cache, the relevant fields is set to None so the next if block will always retrieve from the ORM

[jarekwg]

Looks good. How long are we caching for? Have we got confidence we won't get stuck on old data when extra fields change?

[Sticky-Bits]

Sorry, misread the code.

Yep, this looks good.

[Sticky-Bits]

Looks good. How long are we caching for?

I would assume 30 minutes?

cache.set(cache_key, relevant_fields, 30)

[bayangan1991]

Looks good. How long are we caching for? Have we got confidence we won't get stuck on old data when extra fields change?

Only 30 seconds, which will be refreshed when called within the context of the importer. No where else is set to use the cache at this current stage.

If we set the caching to be the default then the only issue I can see arising is on a particularly active server, the cache will keep getting refreshed before it can expire. This could be resolved by moving the cache.set() to only be called when we fetch from the db so the call will happen every 30 seconds regardless.