Add Change The Nullability Of A Column 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/).
 
-_533 TILs and counting..._
+_534 TILs and counting..._
 
 ---
 
@@ -342,6 +342,7 @@ _533 TILs and counting..._
 - [Attribute Was](rails/attribute-was.md)
 - [Autosave False On ActiveRecord Associations](rails/autosave-false-on-activerecord-associations.md)
 - [Capybara Page Status Code](rails/capybara-page-status-code.md)
+- [Change The Nullability Of A Column](rails/change-the-nullability-of-a-column.md)
 - [Code Statistics For An Application](rails/code-statistics-for-an-application.md)
 - [Conditional Class Selectors in Haml](rails/conditional-class-selectors-in-haml.md)
 - [Convert A Symbol To A Constant](rails/convert-a-symbol-to-a-constant.md)

rails/change-the-nullability-of-a-column.md

@@ -0,0 +1,28 @@
+# Change The Nullability Of A Column
+
+Do you have an existing table with a column that is exactly as you want it
+except that it needs to be changed to either `null: false` or `null: true`?
+
+One option is to use ActiveRecord's `change_column_null` method in your
+migration.
+
+For example to change a nullable column to `null: false`, you'll want a
+migration like the following:
+
+```ruby
+def change
+  change_column_null :posts, :title, false
+end
+```
+
+Note, if you have existing records with `null` values in the `title` column,
+then you'll need to deal with those before migrating.
+
+If you want to make an existing column nullable, change that `false` to
+`true`:
+
+```ruby
+def change
+  change_column_null :posts, :title, true
+end
+```