always gen ecto bool

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content/tils/always_generated_ecto_boolean.md

@@ -0,0 +1,45 @@
+---
+title: Always Generated Boolean tied to a date, smart deleted states in sql
+date: 2023-09-25
+---
+It's a common scenario to not only know that something happened, but also _when_ it happened. Such is the common case with something like the following example of marking a user as `deleted?: true`. But, rather than relying on keeping two attributes up to date (`deleted` and `deleted_at`) we can rely on the database to have an automatically generated boolean to handle that for us.
+
+The following example implements a reversable database migration, field, and function used to mark a user as deleted (at the current timestamp) and undeleted (or reactivated). With all three implemented we'll be able to tell if a user is deleted `user.deleted? == true` and toggle a user's deletion `delete_user(current_user)` and `undelete_user(deleted_user)`.
+
+```elixir
+# mix ecto.gen.migration add_deleted_to_user
+defmodule App.Repo.Migrations.AddDeletedToUser do
+  use Ecto.Migration
+
+  def change do
+    alter table(:users) do
+      add(:deleted_at, :naive_datetime)
+    end
+
+    execute(
+      "alter table users add column deleted boolean generated always as (deleted_at is null) stored",
+      "alter table users drop column deleted"
+    )
+  end
+end
+```
+
+```elixir
+# App.Accounts.User
+field(:deleted?, :boolean, read_after_writes: true, source: :deleted)
+field(:deleted_at, :naive_datetime)
+```
+
+```elixir
+  def delete_user(%User{} = user),
+    do:
+      user
+      |> User.changeset(%{deleted_at: NaiveDateTime.utc_now()})
+      |> Repo.update()
+  
+  def undelete_user(%User{} = user),
+    do:
+      user
+      |> User.changeset(%{deleted_at: nil})
+      |> Repo.update()
+```