Add Write Reversible Migration To Set Default as a rails til

Author: jbranchaud

README.md

@@ -9,7 +9,7 @@ and pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_905 TILs and counting..._
+_906 TILs and counting..._
 
 ---
 
@@ -584,6 +584,7 @@ _905 TILs and counting..._
 - [Upgrading Your Manifest For Sprocket's 4](rails/upgrading-your-manifest-for-sprockets-4.md)
 - [Where Am I In The Partial Iteration?](rails/where-am-i-in-the-partial-iteration.md)
 - [Wipe Out All Precompiled Assets](rails/wipe-out-all-precompiled-assets.md)
+- [Write Reversible Migration To Set Default](rails/write-reversible-migration-to-set-default.md)
 - [Write Safer Where Clauses With Placeholders](rails/write-safer-where-clauses-with-placeholders.md)
 
 ### React

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/)