Thymeleaf: select a variable by another one - thymeleaf

Question

This is my model.
keys: "a", "b", "c"
a: "bla", "bla"
b: "blu", "blu"
c: "hi", ha"
I am not sure how to select the list I like to iterate by the element of another list.
e.g. list x where x is 1st of keys
<select>
<optgroup th:each="k : ${keys}" th:label="${k}">
<option th:each="m : ${${k}}" th:text="${m}"></option>
</optgroup>
</select>
Here it is ${${k}} - how do I express this correct in thymleaf??
EDIT
Because it was asked in the comments...
I have a
Map <String, List <String>>
in Java but was not able to put it into org.springframework.ui.Model.addAttributes ().
Not sure but I think it only allows flat Collections.
So I put the list of keys into the Model and also the list for every key.
I would appreciate if I can do it in a more direct way - but not sure how...

Answer 1

There are a couple ways to do this:
Using the #ctx utility object (may differ slightly depending on your Thymeleaf version):
<select>
<optgroup th:each="k : ${keys}" th:label="${k}">
<option th:each="m : ${#ctx[k]}" th:text="${m}"></option>
</optgroup>
</select>
Preprocessing:
<select>
<optgroup th:each="k : ${keys}" th:label="${k}">
<option th:each="m : ${__${k}__}" th:text="${m}"></option>
</optgroup>
</select>
That being said, I'm not sure you'd want to do it this way -- why not have a Map or some other data structure you've built for this? You can run into runtime errors and/or security vulnerabilities (such as exposing internal variables when you don't want to) when using these kind of methods.