Once an application has been deployed data gets added. Inevitably requirements change. Making schema/data changes to a live database is not an easy task hence ActiveRecord::Migration. This is a great way to make changes to a live database and is pretty much what I was doing at my last company with SQL deltas except AR Migrations go one better by making it database independant.
However, some migrations can be pretty tricky. What I propose is a modification of the test framework to allow it to be run in a non-destructive mode against the production db (or copy of production to test the migration). Specific tests could be written to ensure the migration didn’t adversely affect the live data.
—PhilThompson