The Salesforce CRM application somewhat "famously" does not use native DB foreign keys to model most relationships, and has a custom relational integrity and indexing layer.

Mostly, this is in order to support our complex custom schema functionality, described in the Multitenant Whitepaper here (https://www.developerforce.com/media/ForcedotcomBookLibrary/...)

But interestingly, this also lets our internal data modelers choose to make a relationship e.g. leave dangling records or have other update/delete patterns that are appropriate for the data and data volume. Obviously there are cleanups and tradeoffs required here. It's certainly not a model I'd suggest people start with, but there really are times where database native FKs aren't the answer.