on_load Module callback example (before)

changeset.ex

defmodule MyAppAPI.Changeset do
  import Ecto.Changeset
  
  @known_buckets Application.get_env(:myapp_api, :known_buckets)
  
  # ...
  
  def changeset(schema, attrs) do
    schema
    |> cast(attrs, @fields)
    |> validate_inclusion(:bucket, @known_buckets)
  end
end