Add Columns With Default Values Are Nil On Create as a rails til

Author: jbranchaud

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).
 
-_1015 TILs and counting..._
+_1016 TILs and counting..._
 
 ---
 
@@ -619,6 +619,7 @@ _1015 TILs and counting..._
 - [Check If ActiveRecord Update Fails](rails/check-if-activerecord-update-fails.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)
 - [Comparing DateTimes Down To Second Precision](rails/comparing-datetimes-down-to-second-precision.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/columns-with-default-values-are-nil-on-create.md

@@ -0,0 +1,28 @@
+# Columns With Default Values Are Nil On Create
+
+Let's say I have a `MagicLinks` model backed by `magic_links` Postgres table.
+Both the `id` and `token` columns are of type `UUID` and have default values of
+`gen_random_uuid()`. That means from the Rails-side when I go to create a
+`MagicLink` record, I don't have to think about specifying values for `id` or
+`token` -- the DB will take care of that.
+
+```ruby
+> magic_link = MagicLink.create(expires_at: Time.zone.now, user: User.last)
+  User Load (5.9ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" DESC LIMIT $1  [["LIMIT", 1]]
+  TRANSACTION (0.1ms)  BEGIN
+  MagicLink Create (3.1ms)  INSERT INTO "magic_links" ("user_id", "expires_at", "created_at", "updated_at") VALUES ($1, $2, $3, $4) RETURNING "id"
+...
+
+> magic_link.id
+=> "6c6dddbf-4427-407d-8dc8-eef8cb65d491"
+> magic_link.token
+=> nil
+```
+
+This `create` call is translated into an `insert` SQL statement that includes a
+`returning` clause. For `create` it is always `returning "id"`. This means that
+the `UUID` value generated in Postgres-land for `id` gets passed back into the
+ActiveRecord instance. The `UUID` value generated for `token`,  however, is not
+because `token` isn't specified in the `returning` clause.
+
+[source](https://github.com/rails/rails/issues/17605)