Spring MVC binding to wrong fields

I have a controlle开发者_如何学Cr with 2 methods that return related objects via the @ModelAttribute annotation:

@ModelAttribute("site")
public Site getSite(){
.....
return site;
}

@ModelAttribute("document")
public Document getDocument(){
.....
return document;
}

These objects are related to each other with one Site having many Documents. This relationship is mapped in JPA. Both of these objects contain a field with the same name, called "urlAlias". This field is edited on a page using the following freemarker markup:

<@spring.bind "document" />
....
<@spring.formInput "document.urlAlias" />

When I submit the form to the controller, I retrieve the document object using the following syntax:

@RequestMapping(method = RequestMethod.POST)
    public ModelAndView create(@ModelAttribute("document") @Valid Document document, BindingResult documentResult,
            @ModelAttribute("site") Site site, Model model){
...Do Stuff...
}

It appears that any value that I enter into the Document's urlAlias field has also been set in the Site object, even though I only edited the value of the field in the Document object.

I'm perplexed as to what is going on here. Am I doing something untoward by mapping more than one ModelAttribute in the same controller? Are there any other likely causes of this behaviour?

---

It would appear that the problem is the site parameter in the create() method in my controller:

@ModelAttribute("site") Site site

Removing that stops Spring binding to fields in that object. For future googlers, I get hold of the Site object within the create() method using the code below instead:

if (!model.containsAttribute("site")) {
    throw new IllegalArgumentException("Model must contain site attribute.");
}
Site site = (Site) model.asMap().get("site"); 

From this it would appear that it is fine to declare more than one ModelAttribute in a controller, but only one can be used at a time as a parameter in a method.