The book uses this code in the age group filter:

<form phx-change="age_group_filter" phx-target="<%= @myself%>">
        <label>Filter by age group:</label>
        <select name="age_group_filter" id="age_group_filter">
          <%= for age_group <-
                ["all", "18 and under", "18 to 25", "25 to 35", "35 and up"] do %>
            <option
              value="<%= age_group %>"
              <%=if @age_group_filter == age_group, do: "selected" %> >
                <%=age_group%>
            </option>
          <% end %>
        </select>
      </form>

But I think that there is a more elegant way to do this:

<%= f = form_for :age_group, "#", [phx_change: :age_group_change, phx_target: @myself] %>
    <%= select f, :age_group_filter,
        ["all", "18 and under", "18 to 25", "25 to 35", "35 and up"],
        selected: @age_group_filter
    %>
</form>