Virtual / computed columns in Ecto / Elixir

ecto.ex

  schema "comments" do
    field :text, :string 
    # ... more schema ...
    field :ancestor_id, :integer, virtual: true
  end
  
  def thread(comment) do
    from c in Comment,
      join: cp in CommentPath, on: c.id == cp.descendant_id,
      where: cp.ancestor_id == ^comment.id,
      select: %{c | ancestor_id: cp.ancestor_id}
  end