on_load Module callback example

changeset.ex

defmodule MyAppAPI.Changeset do
  import Ecto.Changeset
  
  @on_load :load_buckets
  
  # ...
  
  def load_buckets do
    known_buckets = "KNOWN_BUCKETS" |> System.get_env() |> String.split(",", trim: true)
    Application.put_env(:myapp_api, :known_buckets, known_buckets)
  end  

  # ...

  def changeset(schema, attrs) do
    known_buckets = Application.get_env(:myapp_api, :known_buckets)
    
    schema
    |> cast(attrs, @fields)
    |> validate_inclusion(:bucket, known_buckets)
  end
end