Deleting an ad zone or category removes all ad impression data from that zone/category

[melinath]

If a website is redesigned and the zones change, right now, all ads that were in the zone will be deleted - and by association, all ad impressions from ads in that zone will be deleted. Which would make ad impression data irrecoverably deleted.

I think there are a couple of potential solutions here.

1. Set ad category/zone foreign keys to NULL on delete instead of cascading.
2. Record advertisers on AdImpressions instead of/in addition to ads. Slash set the AdImpression -> Ad foreign key to NULL on delete instead of cascading.