Add Check If Any Records Have A Null Value as a rails til

rails/check-if-any-records-have-a-null-value.md

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+# Check If Any Records Have A Null Value
+
+I like to add missing `not null` constraints where appropriate when they are
+missing. The [`change_column_null`
+method](change-the-nullability-of-a-column.md) is the Rails DSL way of doing
+this.
+
+But first, you need to be sure that all databases this will be running against
+don't have any `null` values already present for any records. If there are
+`null` values present, then the migration will fail.
+
+One way of doing that check is with some SQL and the
+[`select_values`](https://api.rubyonrails.org/v6.1.0/classes/ActiveRecord/ConnectionAdapters/DatabaseStatements.html#method-i-select_values)
+method.
+
+```ruby
+ActiveRecord::Base
+  .connection
+  .select_values("select id from projects where user_id is null")
+  .any?
+```
+
+This bit of SQL asks for the `id`s of any records in the `projects` table where
+a specific column (`user_id`) is explicitly `null`.
+
+If this returns `false`, then you are good to migrate. If this returns `true`,
+then you'll have to do some data massaging first.