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

README.md

@@ -10,7 +10,7 @@ pairing with smart people at Hashrocket.
 
 For a steady stream of TILs, [sign up for my newsletter](https://tinyletter.com/jbranchaud).
 
-_1074 TILs and counting..._
+_1075 TILs and counting..._
 
 ---
 
@@ -645,6 +645,7 @@ _1074 TILs and counting..._
 - [Change The Nullability Of A Column](rails/change-the-nullability-of-a-column.md)
 - [Change The Time Zone Offset Of A DateTime Object](rails/change-the-time-zone-offset-of-a-datetime-object.md)
 - [Check If ActiveRecord Update Fails](rails/check-if-activerecord-update-fails.md)
+- [Check If Any Records Have A Null Value](rails/check-if-any-records-have-a-null-value.md)
 - [Check Specific Attributes On ActiveRecord Array](rails/check-specific-attributes-on-activerecord-array.md)
 - [Code Statistics For An Application](rails/code-statistics-for-an-application.md)
 - [Columns With Default Values Are Nil On Create](rails/columns-with-default-values-are-nil-on-create.md)

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

@@ -0,0 +1,27 @@
+# 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.