"${#fields.hasErrors('*')}" always false - Redirect Problem - forms

Question

My controller looks like this:
#PostMapping("/event/{id}")
public String save(#PathVariable("id") long id, #Valid Form form, BindingResult bindingResult ) {
if (!bindingResult.hasErrors()) {
//No errors
//No return
}
return "redirect:/event/{id}";
}
My #GetMapping is:
#GetMapping("/event/{id}")
public ModelAndView eventDetail(#PathVariable("id") long id) {
ModelAndView model = new ModelAndView("event/details");
Event event = eventoRepository.findById(id).get();
model.addObject("event", evento);
model.addObject("guests", event.getGuests());
model.addObject("guest",new Guest());
return model;
}
I know that "${#fields.hasErrors('*')}" is always false because redirect. (right?)
How return to this path /event/{id} without redirect?

Answer 1

I know that "${#fields.hasErrors('*')}" is always false because redirect. (right?)
Right. It looks like you always redirect. Because of that a method annotated with #GetMapping("/event/{id}") is called, the form most likely is reseted to fresh state and there are no more errors making expression always false.
How return to this path /event/{id} without redirect?
Simply return name of the view (template) containing the form. Most likely it's the same what's returned by method annotated with #GetMapping("/event/{id}").
You should follow an approach from this guide. Return without redirect if the form contains error, and redirect otherwise.
Edit:
You should also provide additional objects to the model. Instead of populating model in each method (Get, Post etc.) you may extract common objects to a method annotated with #ModelAttribute. According to javadoc such a method can accept similar parameters as methods annotated with #RequestMapping.
For your case something like this should work fine:
#ModelAttribute
void supplyModel(#PathVariable("id") long id, Model model) {
Event event = eventoRepository.findById(id).get();
model.addAttribute("event", evento);
model.addAttribute("guests", event.getGuests());
model.addAttribute("guest",new Guest());
}
#GetMapping("/event/{id}")
public String eventDetail(#PathVariable("id") long id) {
return "event/details";
}
#PostMapping("/event/{id}")
public String save(#PathVariable("id") long id, #Valid Form form, BindingResult bindingResult ) {
if (bindingResult.hasErrors()) {
// Has errors
return "event/details";
}
// No errors
return "redirect:/event/" + id;
}