Assign an object in a form - thymeleaf

Question

I have a form which should print questions dynamically. Foo has a Field object and a Field Definition object. I want the Field to have its fieldDefinition assigned by the form. All of the behind the scenes stuff works fine.
The below code works with assigning Strings and Longs in other scenarios.
Here's the line that's causing trouble:
<input th:type="hidden" th:field="*{fields[__${iterationStatus.index}__].fieldDefinition}" th:value="${fooViewModel.fields[__${iterationStatus.index}__].fieldDefinition}"/>
This is what it looks like when it renders in html:
<input type="hidden" value="com.blah.domain.FieldDefinition#fbb2e392" id="fields0.fieldDefinition" name="fields[0].fieldDefinition">
When I submit the form, no controller action is invoked, and the app simply redirects to the error page.
If it's impossible to actually do the assignment that way, please suggest other methods. The only way I came up with is to have Foo use the FieldDefinitionService to do the assignment after being passed an ID. I don't that Domain class to have access to another Domain object's Service.
Thanks

Answer 1

Just an example: when you need to iterate a form inside an element that has a th:each attribute, you can use the following structure (so far, it's the only way that's working for me).
<tr th:each="rank, stat : ${ranks}">
<td th:text="${rank.name}">This is static</td>
<td th:text="${rank.description}">This is static</td>
<td>
<form th:action="#{/user/ranks/delete}" method="post">
<input type="hidden" id="id" name="id" th:value="${rank.id}"></input>
<button class="btn btn-danger" type="submit">
<span>Delete</span>
</button>
</form>
</td>
</tr>
Here ranks is a list of entities that have to be displayed on a table and, for each entity, there is a delete button associated to a form.
The controller method should be similar to the following fragment, the parameter is availabled with the name id:
#RequestMapping(path = "/delete", method = RequestMethod.POST)
public View deleteRank(Model model, #RequestParam(name = "id") String rankId,
#ModelAttribute("user") User user)
{
Long id = Long.parseLong(rankId);
// delete ...
RedirectView redirectView = new RedirectView("/user/ranks");
return redirectView;
}