Add Change The Nullability Of A Column as a rails til

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

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+# 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
+```