Testing with a DB I don't control.

The Rails framework, for testing, seems to assume that the app creates/defines the
database it uses.  But what about when I'm only *reading* an existing DB? 

Granted, I could write migrations that *approximate* the schema of the existing DB, and then
populate the test DB (again, in an approximate way) in my tests but that's not very helpful.

How do *you* write and run tests when you don't want to erase the test DB?

Thanks,

Mike