In this talk on Ecto the speaker recommends to calculate persistent fields from virtual Ecto.Schema fields that come in from user input in a changeset.

In his example there is an Ecto.Schema like

defmodule MusicDb.Track
...
  schema "tracks" do
    field :title, :string
    field :duration, :integer
    field :duration_string, :string, virtual: true

    timestamps()
  end

The duration field holds the duration in seconds persistently in the database. The duration_string is used to hand in the duration time from user input as a string, For example duration_string comes in as "3:25" in the changeset which would be piped through a function that sets the time_in_seconds field to 205.

So far so good. But how do I go the way back? When I query a Track my duration_string field would be nil. Is there any way to install some hook function in the schema where the Track struct is piped through whenever I query a Track so that I can calculate and set duration_string?