Display Thymeleaf server side field validation error with bootstrap

<form action="#" th:action="@{/}" th:object="${myForm}" method="post"> <!-- 'myform' is the form object -->
        <!-- start row 1 -->
        <div class="form row">
            <!-- col 1 -->
            <div class="col-lg-6">
                <div class="form-group">
                    <label th:for="*{firstname}">First name</label>
                    <input th:field="*{firstname}" type="text" class="form-control"
                           th:classappend="${#fields.hasErrors('firstname')}? is-invalid"  <!-- adds this class in case there is a field error-->
                           placeholder="first name"
                           aria-describedby="firstnameFeedback"> <!-- see server-side validation from Bootstrap docs -->
                    <div id="firstnameFeedback" th:if="${#fields.hasErrors('firstname')}"
                         th:errors="*{firstname}" class="invalid-feedback"> <!-- is initially not displayed -->
                    </div>
                </div>
                (...)