checksum

A number that changes when this migration changes, or null - the default - for "do not check".

There is no honest checksum to derive for a class. Hashing the bytecode reports a change when a blank line moves or the compiler is upgraded, and hashing the class name detects only a rename, which the version already catches. So nothing is stored unless you put something here, and validation skips what it has no checksum for.

On a repeatable migration null means it runs on every run, there being no way to tell that it changed - which is no great loss, a repeatable migration having to be idempotent regardless.