Ecto validation for non-nil embedded schema

foo.ex

defmodule MyApp.Data.Schemas.Foo do
  use Ecto.Schema
  
  import Ecto.Changeset
  import MyApp.Data.Schemas.PrepareEmbed

  alias MyApp.Data.Schemas.EmbeddedMap
    
  schema "foo" do
    embeds_one :embedded_map, EmbeddedMap, on_replace: :update
  end
  
  def changeset(item, attrs) do
    item
    |> cast(attrs, [])
    |> prepare_embed(:embedded_map)
    |> cast_embed(:embedded_map) 
  end
end

prepare_embed.ex

defmodule MyApp.Data.Schemas.PrepareEmbed do
  def prepare_embed(%Ecto.Changeset{data: data, params: params} = change, field) do
    with f <- to_string(field) do
      case {Map.get(params, f), Map.get(data, f)} do
        {nil, nil} ->
          {:embed, field_spec} = change.data.__struct__.__schema__(:type, field)
          params = Map.put(params, f, field_spec.related.__struct__ |> Map.from_struct())
          Map.put(change, :params, params)

        _ ->
          change
      end
    end
  end
end