I work with a lot of web applications that are driven by databases of varying complexity on the backend. Typically, there's an [ORM](http://en.wikipedia.org/wiki/Object-relational_mapping) layer separate from the business and presentation logic. This makes unit-testing the business logic fairly straightforward; things can be implemented in discrete modules and any data needed for the test can be faked through object mocking.

But testing the ORM and database itself has always been fraught with problems and compromises. 

Over the years, I have tried a few strategies, none of which completely satisfied me.

* Load a test database with known data. Run tests against the ORM and confirm that the right data comes back. The disadvantage here is that your test DB has to keep up with any schema changes in the application database, and might get out of sync. It also relies on artificial data, and may not expose bugs that occur due to stupid user input. Finally, if the test database is small, it won't reveal inefficiencies like a missing index. (OK, that last one isn't really what unit testing should be used for, but it doesn't hurt.)
* Load a copy of the production database and test against that. The problem here is that you may have no idea what's in the production DB at any given time; your tests may need to be rewritten if data changes over time.

Some people have pointed out that both of these strategies rely on specific data, and a unit test should test only functionality. To that end, I've seen suggested:

* Use a mock database server, and check only that the ORM is sending the correct queries in response to a given method call.

What strategies have you used for testing database-driven applications, if any? What has worked the best for you?