Grouped_select, how to display previously selected item and/or category

In a reservation form, I'm trying to show available rooms and their room_category if they are available for a specific time span. Everything works fine when creating a new reservation.

Issue

When I want to edit the reservation form (and more specifically the time span), the form drop-down list empties and gets build up again. The issue that arrises is that the previously chosen room and its room_category are not shown as selected when they are available.

Question

How to:

• select the previously picked room if it's available for the newly selected dates
• alternatively, if the room is not available, how to automatically select a room from the previously chosen room_category

Code

reservation form

<div class="col col-sm-4">
        <%= f.input :arrival,
        as: :string,
        label:false,
        placeholder: "From",
        wrapper_html: { class: "inline_field_wrapper" },
        input_html:{ id: "start_date"} %>
      </div>
      <div class="col col-sm-4">
        <%= f.input :departure,
        as: :string,
        label:false,
        placeholder: "From",
        wrapper_html: { class: "inline_field_wrapper" },
        input_html:{ id: "end_date"} %>
      </div>

      <div class="col col-sm-4">
        <%= f.input :room_id, collection: @room_categories.order(:name), as: :grouped_select, group_method: :rooms, label:false %>
      </div>

JS

const checkIn = document.querySelector('#start_date');
const checkOut = document.querySelector('#end_date');


  const checkInAndOut = [checkIn, checkOut];

  checkInAndOut.forEach((item) => {
    item.addEventListener('change', (event) => {
      if ((checkIn.value.length > 0) && (checkOut.value.length > 0)){
      checkAvailability();
    }
  })
});

    function checkAvailability(){

      Rails.ajax({
        url: "<%= rooms_availability_hotel_path(@hotel) %>" ,
        dataType: 'json',
        type: "POST",
        data: `arrival=${start_date.value}&departure=${end_date.value}`,
        success: function(data) {
          console.log(data);
        },
        error: function(response) {
          console.log(response);
        }
      });
    };

controller check availability

  def rooms_availability
    hotel = Hotel.includes(:rooms).find(params[:id])
    arrival = Date.parse(room_params[:arrival])
    departure = Date.parse(room_params[:departure])
    time_span = arrival..departure
    @unavailable_rooms = Room.joins(:reservations).where(reservations: {hotel: hotel}).where("reservations.arrival <= ? AND ? >= reservations.departure", arrival, departure).distinct + (Room.joins(:reservations).where(reservations: {hotel: hotel}).where("reservations.arrival <= ? AND ? <= reservations.departure", arrival, departure).distinct)
    @hotel_cats = hotel.room_categories
    @hotel_accos = Room.where(room_category: @hotel_cats)
    @rooms = @hotel_accos - @unavailable_rooms
    respond_to do |format|
      format.js
    end
  end

hotels/rooms_availability.js.erb

var selectList = document.getElementById('reservation_room_id')
function empty() {
  selectList.innerHTML = "";
}

empty();

<% unless @rooms.empty? %>
      <% @hotel_cats.each do |cat|%>
        selectList.insertAdjacentHTML('beforeend', '<optgroup label=<%= cat.name %>>');
        <% cat.rooms.each do |room|%>
          <% if @rooms.include? room %>
            selectList.insertAdjacentHTML('beforeend', '<option value="<%= room.id %>"><%= room.name %></option>');
          <% end %>
        <% end %>
        selectList.insertAdjacentHTML('beforeend', '<optgroup>');
      <% end %>
    <% end %>

select the previously picked room if it's available for the newly selected dates

<% unless @rooms.empty? %>
<% @hotel_cats.each do |cat|%>
selectList.insertAdjacentHTML('beforeend', '<optgroup label=<%= cat.name %>>');
<% cat.rooms.each do |room|%>
<% if @rooms.include? room %>
selectList.insertAdjacentHTML(
'beforeend',
<%= "<option value='#{room.id}' #{@previous_room.id == room.id ? "selected" : ""}>#{room.name}</option>" %>
);  
<% end %>
<% end %>
selectList.insertAdjacentHTML('beforeend', '<optgroup>');
<% end %>
<% end %>

alternatively, if the room is not available, how to automatically select a room from the previously chosen room_category

<% unless @rooms.empty? %>
<%
is_previous_room_available = @rooms.include? @previous_room
set_selected = false
%>
<% @hotel_cats.each do |cat|%>
selectList.insertAdjacentHTML('beforeend', '<optgroup label=<%= cat.name %>>');
<% cat.rooms.each do |room|%>
<% if @rooms.include? room %>
<%
if (is_previous_room_available)
# When previous room is available
%>
selectList.insertAdjacentHTML(
'beforeend',
<%= "<option value='#{room.id}' #{@previous_room.id == room.id ? "selected" : ""}>#{room.name}</option>" %>
);
<%
elsif (is_previous_room_available.! && set_selected.! && @previous_cat.id == cat.id)
# When previous room not available and this room is first room of previous category
%>
selectList.insertAdjacentHTML(
'beforeend',
<%= "<option value='#{room.id}' selected>#{room.name}</option>" %>
);
<%
# set first room is selected
set_selected = true
else
%>
selectList.insertAdjacentHTML(
'beforeend',
<%= "<option value='#{room.id}'>#{room.name}</option>" %>
);
<%
end
%>
<% end %>
<% end %>
selectList.insertAdjacentHTML('beforeend', '<optgroup>');
<% end %>
<% end %>