I have a multiple_select to select a single medic for an appointment, I managed to show up a list to choose a medic, and when inspecting the element it does have an id as a value, the thing is that when trying to add an appointment it will give an error is invalid in the medic field.
This is my form:

<%= form_for @changeset, @action, fn f -> %>
  <%= if @changeset.action do %>
    <div class="alert alert-danger">
      <p>Oops, something went wrong! Please check the errors below.</p>
    </div>
  <% end %>

(...)

  <div class="form-group">
    <%= label f, :id_medico, class: "control-label" %>
    <%= multiple_select f, :id_medico, Enum.map(@medics, &{&1.nombre, Integer.to_string(&1.id)}) %>
    <%= error_tag f, :id_medico %>
  </div>

(...)

  <div class="form-group">
    <%= submit "✔", class: "btn btn-primary" %>
    <a class="btn btn-danger" href="/appointments" role="button">✖</a>
  </div>
<% end %>

and my controller:

  def new(conn, _params) do
    medics = Clinic.list_medics()
    changeset = Clinic.change_appointment(%Appointment{})
    render(conn, "new.html", changeset: changeset, medics: medics)
  end

Thanks.