I cannot test it now, but I think you need to call to_conversation/1 outside the Memento transaction.

This is how I do in my app…

To get a resource:

def get_todo!(uid) do
    _read_transaction(uid)
  end

To read it from Mnesia:

defp _read_transaction(uid) do
    result = Memento.transaction fn -> Memento.Query.read(Todo, uid) end
    _parse_fetch_result(result)
end

To parse the result of reading it from Mnesia:

defp _parse_fetch_result(result) do
    case result do
      {:ok, nil} ->
        []
      {:ok, []} ->
        []
      {:ok, todo} ->
        todo
      {:error, error} ->
        Logger.error("_parse_resul/1: #{error}")
        Todo.new_changeset()
    end
  end

To note that the returned values foreach case are specific to my app.

Also bear in my mind that is app serves as a Learning path for me to know more about Mnesia and Elixir, thus I may also not doing it in the best way