Personally I usually end up changing the migration itself to put in the correct data. It never feels like the right way of doing things, though it does work.
Another solution is to define the model in your migration:
class MigrationStuff …
class MyModel < AR::Base
That way you aren’t using the validations, and in the migration were validations are added, you can run through the existing data and make changes to be sure it won’t break the app further down the line.