Add Migrating Up Down Up as a rails til.

rails/migrating-up-down-up.md

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+# Migrating Up Down Up
+
+When writing Rails migrations, it is good to define, when possible, what
+should happen when migrating *up* and what should happen when migrating
+*down*. You'll then want to check that both the *up* and *down* work. This
+can be accomplished using the following one-liner:
+
+```bash
+$ rake db:migration && rake db:migration:redo
+```
+
+The `rake db:migration` does what we would expect applying our new migration
+and showing us that our *up* works. The `rake db:migrate:redo` first
+performs a rollback, showing us that our *down* works, and then migrates
+back up again. We now know that our latest migration works going both
+directions.