Add Write Reversible Migration To Set Default as a rails til

rails/write-reversible-migration-to-set-default.md

@@ -0,0 +1,29 @@
+# Write Reversible Migration To Set Default
+
+You can use the `change_column_default` method to alter the default value of a
+column. If the column doesn't have a default, then you'd essentially be
+changing the default from `nil` to _some value_.
+
+```ruby
+def up
+  change_column_default :books, :published, false
+end
+
+def down
+  change_column_default :books, :published, nil
+end
+```
+
+This is fine, but you can write the migration as a single, reversible `change`
+method using the `:from` and `:to` options.
+
+```ruby
+def change
+  change_column_default :books, :published, from: nil, to: false
+end
+```
+
+When you migrate, the default will be set to `false`. When you rollback, the
+default will be removed.
+
+[source](https://blog.arkency.com/how-to-add-a-default-value-to-an-existing-column-in-a-rails-migration/)