How to update an element in spring boot using save method?

I have two classes which have the following elements:

@Document(collection = "ClassP")
Class P{
@Id
private String p_id;
}

 @Document(collection = "ClassR")
Class R{
@Id
private String r_id;
private String item;

@DBRef
private P p;

@DBRef
private User user;
}

Here is my PRepository:

public interface PMongoRepository extends CrudRepository<P, String>{

P findPById(String p_id);
}

What I am going to do is to update the item from class R. I get the changed item from the frontend and in my controller I have the new item. So, there is no problem from the front end side.
In my controller, I have the following code:

@RequestMapping(value = "/editData", method = RequestMethod.POST, consumes = "application/json")
	public ModelAndView editData(@RequestBody Map<String, String> new_items) {

		ModelAndView modelAndView = new ModelAndView();

		R copyOfExistingR= new R();

		P pFound = pRepository.findPById(new_items.get("p_id"));
			copyOfExistingR.setP(pFound);
            copyOfExistingR.setItem(new_items.get("sep"));
            copyOfExistingR.setUser(user);
            rRepository.save(copyOfExistingR);

return modelAndView ;
}

But the code doesn't work as expected. On the rRepository.save(copyOfExistingR); I get the following error:

Cannot create a reference to an object with a NULL id.

The pFound is not null and I can print it out, but I don't know what I am doing wrong with updating the R class. I would be thankful if anybody could help me.

You never set id of copyOfExistingR.

The solution is that you just set the 'r_id'

@RequestMapping(value = "/editData", method = RequestMethod.POST, consumes = "application/json")
        public ModelAndView editData(@RequestBody Map<String, String> new_items) {
    
            ModelAndView modelAndView = new ModelAndView();
    
            R copyOfExistingR = new R();

          //-----Here you set your r_id for object copyOfExistingR----
    
            P pFound = pRepository.findPById(new_items.get("p_id"));
                copyOfExistingR.setP(pFound);
                copyOfExistingR.setItem(new_items.get("sep"));
                copyOfExistingR.setUser(user);
                rRepository.save(copyOfExistingR);
    
    return modelAndView ;
    }

The problem was that I had defined a variable above @Id in the P class and it was problematic. The @Id must always be the first item in the class.