thymeleaf replacing th:field to comply with th:selected - spring-boot

Question

I appreciate your passing by my post. I have searched here in StackOverFlow and google as well to fix my following code:
My HTML code:
<form th:action="#{/surgerywaitinglist/saveToWaitinList}"
th:object="${waitinglistDTO}" method="POST">
.
.
.
<select name="departmentName"
th:field="*{department.departmentName}"
th:with="departmentName = ${waitinglistDTO.department.departmentName}"
class="form-control" id="departmentJS">
<option value="" th:selected="selected"
th:disabled="disabled">select option
</option>
<option th:each="department: ${departments}"
th:value="${department.departmentName}"
th:text="${department.departmentName}">
</option>
</select>
.
.
.
<input type="submit" value="Save" class="btn btn-primary" />
</form>
form other posts like this post I found out that th:field and th:selected do not work together; in fact, th:field needs to be replace with something else. Notice that the th:object holds another object (from Department class) ...
My DTO class:
public class WaitinglistDTO {
private Long waitingListId;
#NotBlank(message = "Please enter a procedure")
private String waitingListProcedure;
private String waitingListDiagnosis;
private Long waitingListPatientId;
private Long waitingListSurgeonId;
private Long waitingListDepartmentId;
#DateTimeFormat(iso=ISO.DATE)
private Date waitingListAdditionDate;
#DateTimeFormat(iso=ISO.DATE)
private Date waitingListActualBookingDate;
private Patient patient;
private Surgeon surgeon;
private Department department;
Could you help me figure this out?
Many thanxxxxx :)
...
Update:
The following image explains how it should look,,, however, the default option should be the select option which should be somehow disabled
This is the result I get when I apply the suggested code of Rafael da Silva ,, you can see the pre-selected option is the first option rather than the select option option :)

Answer 1

you can do like this:
<select name="departmentName" th:field="*{department.departmentName}" th:with="departmentName = ${waitinglistDTO.department.departmentName}" class="form-control" id="departmentJS">
<option selected="selected" disabled="disabled">select option</option>
<option th:each="department : ${departments}" th:value="${department.departmentName}" th:text="${department.departmentName}"></option>
</select>
this worked for me.

Answer 2

I don't see why you would add the selected to the blank option, the first option is always selected if your model object has nothing set and if it does normal behaviour would be to have that item selected, which th:field should resolve.
In case you want the select to always start at the default option, even if the model has another value or want to set it some other way. you can set th:id="*{department.departmentName}" and th:name="*{department.departmentName}" and than manualy handle th:selected.
As a side note theres no need to use the th version th:selected="selected" if using static values just selected or selected="selected would sufice in that case
Edit with code, not tested as i don't have my work pc where i am.
If you want to always have the first option selected even when editing with previous data
<select name="departmentName" th:name="*{department.departmentName}" th:id="*{department.departmentName}" class="form-control" id="departmentJS">
<option selected disabled>select option</option>
<option th:each="department : ${departments}" th:value="${department.departmentName}" th:text="${department.departmentName}"></option>
</select>
If you want to fill with existing data but default to the first option
<select name="departmentName" th:field="*{department.departmentName}" class="form-control" id="departmentJS">
<option disabled>select option</option>
<option th:each="department : ${departments}" th:value="${department.departmentName}" th:text="${department.departmentName}"></option>
</select>
I'm not 100% sure on the use of disabled but i think it should work, personally i would just leave it selectable and enforce a #notnull on your bean in controller validation.
the th:with annotation does nothing asfar i can see, what th:with is used for is defining a new variable for all elements nested inside the elment you define it on. example:
<div th:with="newVar='someString'">
<span th:text="${newVar}"></span>
</div>