set esel_legacy_id from legacy_event_id

use QuickAlias, MyAppWeb
use QuickAlias, MyAppBe
use QuickAlias, MyApp
IEx.configure(inspect: [charlists: :as_lists])
import Ash.Query

Section

query =
  MyApp.Event
  |> filter(legacy_event_id > 1 and is_nil(esel_legacy_id))

query
|> Ash.count!()

query
|> limit(10000)
|> Ash.read!()
|> Enum.map(fn rec ->
  rec
  |> Ash.Changeset.for_update(:update, %{esel_legacy_id: rec.legacy_event_id})
  |> Ash.update!()
end)