Null fields after form submit in Spring - forms

Question

I've got a Product with a Rating rating attribute. I've got a product update form (updateStart method) which doesn't contain the rating field (since I don't want it to be editable).
The problem is that when I submit the form (with update method), the rating is automatically set to null.
So I tried to add the Rating to the form model in updateStart, retrieving it in the update method, but it keeps being rewritten as well.
I tried to set a #SessionAttributes("rating") annotation in the controller. This time the rating value is kept, but Spring creates a new entry in the database, cloned from the other rating object, and attaches it to the Product.
#Controller
#SessionAttributes("rating")
#RequestMapping("/products")
public class ProductsController {
#RequestMapping("/update_start")
public String updateStart(#RequestParam("id") Long id, Model model) throws BusinessException {
Product product = productService.findProductById(id);
System.out.println("RATING A START "+product.getRating().getAbsoluteRating());
List<Category> categories = productService.findAllCategories();
model.addAttribute("categories", categories);
model.addAttribute("product", product);
model.addAttribute("id", id);
model.addAttribute("rating",product.getRating());
return "products.updateform";
}
#RequestMapping(value="/update", method = RequestMethod.POST)
public String update(#ModelAttribute("rating") Rating rating, #ModelAttribute Product product, BindingResult bindingResult) throws BusinessException {
System.out.println("RATING A UPDATE "+rating.getAbsoluteRating());
validator.validate(product, bindingResult);
List<Image> images = imageService.getProductImages(product.getId());
product.setRating(rating);
productService.updateProduct(product,images,sellerid);
return "redirect:/products/viewsforsellers.do";
}
}
What can I do?
EDIT: I'd prefer to avoid placing a hidden input field with ratingId in my form.

Answer 1

In the form include a hidden input with the name and value specified for the Rating. The value should include
<form>
<input name="product.rating" value="${product.rating.id}"/>
<!-- Other fields -->
</form>
Now when the request comes over the wire it should include a Rating specified by id for the product.
#RequestMapping(value="/update", method = RequestMethod.POST)
public String update(#ModelAttribute Product product, BindingResult bindingResult) throws BusinessException {
//implementation
}
#ModelAttribute should attempt to bind this parameter to the Product however it is not aware of what a Rating is. This is where a Converter comes into play. A Converter is used during databinding to tell Spring MVC how to map a field of type String to a field of type Rating.
public class StringToRatingConverter implements Converter<String, Rating> {
public Rating convert(String source) {
//Use the source String to convert to rating
//Possibly via database call or enum conversion, pending ratings type and definition
//Ultimately the code needs to return the appropriate object of type Rating
return rating; //The above implementation will create the rating object.
}
}
The StringToRatingConverter must then be registered in the dispatcher configuration file.
<!-- Register Converters - Used for data binding-->
<bean id="conversionService" class="org.springframework.context.support.ConversionServiceFactoryBean">
<property name="converters">
<list>
<bean class="fully.qualified.path.to.StringToRatingConverter"/>
</list>
</property>
</bean>
The first time I encountered this scenario, I captured it in a post on my blog, which you may be helpful.

Answer 2

You should add "types" element to your #SessionAttributes("rating") annotation in order properties of attributes to be kept; e.g.
#SessionAttributes(types = Rating.class, names = "rating")