columns-with-default-values-are-nil-on-create.md

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.

> 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.

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