Add Mark A Migration As Irreversible as a rails til

README.md

@@ -10,7 +10,7 @@ smart people at [Hashrocket](http://hashrocket.com/).
 For a steady stream of TILs from a variety of rocketeers, checkout
 [til.hashrocket.com](https://til.hashrocket.com/).
 
-_811 TILs and counting..._
+_812 TILs and counting..._
 
 ---
 
@@ -497,6 +497,7 @@ _811 TILs and counting..._
 - [Hash Slicing](rails/hash-slicing.md)
 - [Ignore Poltergeist JavaScript Errors](rails/ignore-poltergeist-javascript-errors.md)
 - [List The Enqueued Jobs](rails/list-the-enqueued-jobs.md)
+- [Mark A Migration As Irreversible](rails/mark-a-migration-as-irreversible.md)
 - [Make ActionMailer Synchronous In Test](rails/make-action-mailer-synchronous-in-test.md)
 - [Mark For Destruction](rails/mark-for-destruction.md)
 - [Migrating Up Down Up](rails/migrating-up-down-up.md)

rails/mark-a-migration-as-irreversible.md

@@ -0,0 +1,29 @@
+# Mark A Migration As Irreversible
+
+It is in your best interest to, as much as is possible, write your Rails
+migrations in a way that they can be safely and reliably rolledback. You want
+your `down` to mirror your `up`, in case anything goes wrong.
+
+This isn't always possible though. There are some migrations, in particular
+data migrations, that cannot be undone. Something is being changed or destroyed
+in an unrecoverable way. When this is the case, you should, by convention,
+raise an `IrreversibleMigration` exception.
+
+```ruby
+class DestructiveMigration < ActiveRecord::Migration[5.2]
+  def up
+    execute "-- some destructive SQL"
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end
+```
+
+If anyone ever tries to rollback this migration, they will see the exception.
+It will be a signal that some manual work is needed to continue rolling back.
+
+See the
+[docs](https://api.rubyonrails.org/classes/ActiveRecord/Migration.html#class-ActiveRecord::Migration-label-Irreversible+transformations)
+for more details.