Add Migrating Up Down Up as a rails til.

README.md

@@ -175,6 +175,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 - [Creating Records of Has_One Associations](rails/creating-records-of-has-one-associations.md)
 - [Hash Slicing](rails/hash-slicing.md)
 - [Ignore Poltergeist JavaScript Errors](rails/ignore-poltergeist-javascript-errors.md)
+- [Migrating Up Down Up](rails/migrating-up-down-up.md)
 - [Params Includes Submission Button Info](rails/params-includes-submission-button-info.md)
 - [Pretend Generations](rails/pretend-generations.md)
 - [Retrieve An Object If It Exists](rails/retrieve-an-object-if-it-exists.md)

rails/migrating-up-down-up.md

@@ -0,0 +1,16 @@
+# 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.