Select options based on other selection

Hello everyone,
i have two html select elemets.

The first select is shows all items of a collection.

Now the second select element should show all elements of a collection based on the selected id of the first select element.
How can i do that?

First select element:

<select class="form-control">
    {{#each getFloors}}
        <option value="{{_id}}">{{name}}</option>

Second select element:

<select class="form-control">{
    {#each getRoomsByFloorId "**ID OF THE FIRST SELECTION**"}}
        <option value="{{_id}}">{{name}}</option>

Do you have any ideas?

Use a ReactiveVar that tracks the selection of the first select. Something like this

<template name="Floors">
 <select class="form-control floors">
    {{#each floor in getFloors}}
        <option value="{{floor._id}}">{{}}</option>
 <select class="form-control rooms">{
    {#each room in getRoomsByFloorId floorId}}
        <option value="{{room._id}}">{{}}</option>
 this.floorId = new ReactiveVar(null);
 'change select.floors'(event, instance) {
  return Template.instance().floorId.get();
 getRoomsByFloorId(floorId) {