Not-null property references a null or transient value - Spring Boot - Thymeleaf

I have a problem with the update on my webapp.

Everytime I try to update an Element i got this error:
not-null property references a null or transient value : me.lucacirfeta.model.Element.user

To solve this problem i need to add on inside my form an input hidden with all the variable of User model, example:

<input th:field="${element.user.id}" class="form-control" type="text" readonly />
<input th:field="${element.user.username}" class="form-control" type="text" readonly />
<input th:field="${element.user.firstName}" class="form-control" type="text" readonly />
<input th:field="${element.user.lastName}" class="form-control" type="text" readonly />
<input th:field="${element.user.password}" class="form-control" type="text" readonly />

But i don't want to add on my form this hidden data.
I also tried to remove in the Element model "nullable = false" in the JoinColumn annotation but when i try to update Hibernate lost the reference on User setting it up to "null".

How can I resolve?

This is my code:

Model

package me.lucacirfeta.model;
// default package

// Generated 29-lug-2020 10.31.08 by Hibernate Tools 4.3.5.Final

import java.util.Date;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.OneToMany;
import javax.persistence.Table;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

/**
 * Element generated by hbm2java
 */
@Entity
@Table(name = "ELEMENT", schema = "TEST")
public class Element implements java.io.Serializable {

	/**
	 * 
	 */
	private static final long serialVersionUID = 7331854204646419731L;
	
	private long id;
	private ElDate elDate;
	private Period period;
	private User user;
	private PlaceOfDiscovery placeOfDiscovery;
	private ElType elType;
	private ElDimension elDimension;
	private String description;
	private String otherDetails;
	private Date created;
	private boolean validation;
	private Date discoveryDate;
	
	private Set<ElementImage> elementImages = new HashSet<ElementImage>(0);

	public Element() {
	}

	@Id
	@Column(name = "ID", unique = true, nullable = false, precision = 22, scale = 0)
	@GeneratedValue(strategy = GenerationType.IDENTITY)
	public long getId() {
		return this.id;
	}

	public void setId(long id) {
		this.id = id;
	}

	@ManyToOne(fetch = FetchType.LAZY, cascade = {CascadeType.ALL})
	@JoinColumn(name = "ID_DATE", nullable=false, insertable=false)
	public ElDate getElDate() {
		return this.elDate;
	}

	public void setElDate(ElDate elDate) {
		this.elDate = elDate;
	}

	@ManyToOne(fetch = FetchType.LAZY, cascade = {CascadeType.ALL})
	@JoinColumn(name = "ID_PERIOD", nullable=false, insertable=false)
	public Period getPeriod() {
		return this.period;
	}

	public void setPeriod(Period period) {
		this.period = period;
	}

	@ManyToOne(fetch = FetchType.LAZY, cascade = {CascadeType.ALL})
	@JoinColumn(name = "ID_USER", nullable=false, insertable=false)
	public User getUser() {
		return this.user;
	}

	public void setUser(User user) {
		this.user = user;
	}

	@ManyToOne(fetch = FetchType.LAZY, cascade = {CascadeType.ALL})
	@JoinColumn(name = "ID_PLACE_OF_DISCOVERY")
	public PlaceOfDiscovery getPlaceOfDiscovery() {
		return this.placeOfDiscovery;
	}

	public void setPlaceOfDiscovery(PlaceOfDiscovery placeOfDiscovery) {
		this.placeOfDiscovery = placeOfDiscovery;
	}

	@ManyToOne(fetch = FetchType.LAZY, cascade = {CascadeType.ALL})
	@JoinColumn(name = "ID_EL_TYPE", nullable=false, insertable=false)
	public ElType getElType() {
		return this.elType;
	}

	public void setElType(ElType elType) {
		this.elType = elType;
	}

	@ManyToOne(fetch = FetchType.LAZY, cascade = {CascadeType.ALL})
	@JoinColumn(name = "ID_EL_DIMENSION", nullable=false, insertable=false)
	public ElDimension getElDimension() {
		return this.elDimension;
	}

	public void setElDimension(ElDimension elDimension) {
		this.elDimension = elDimension;
	}

	@Column(name = "DESCRIPTION")
	public String getDescription() {
		return this.description;
	}

	public void setDescription(String description) {
		this.description = description;
	}

	@Column(name = "OTHER_DETAILS")
	public String getOtherDetails() {
		return this.otherDetails;
	}

	public void setOtherDetails(String otherDetails) {
		this.otherDetails = otherDetails;
	}

	@Column(name = "CREATED")
	@Temporal(TemporalType.TIMESTAMP)
	public Date getCreated() {
		return this.created;
	}

	public void setCreated(Date created) {
		this.created = created;
	}

	@Column(name = "VALIDATION", nullable = false, precision = 1, scale = 0)
	public boolean isValidation() {
		return this.validation;
	}

	public void setValidation(boolean validation) {
		this.validation = validation;
	}

	@Temporal(TemporalType.DATE)
	@Column(name = "DISCOVERY_DATE", length = 7)
	public Date getDiscoveryDate() {
		return this.discoveryDate;
	}

	public void setDiscoveryDate(Date discoveryDate) {
		this.discoveryDate = discoveryDate;
	}

	@OneToMany(fetch = FetchType.LAZY, mappedBy = "element", cascade = {CascadeType.ALL})
	public Set<ElementImage> getElementImages() {
		return this.elementImages;
	}

	public void setElementImages(Set<ElementImage> elementImages) {
		this.elementImages = elementImages;
	}
	
}

Controller

package me.lucacirfeta.controllers;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.servlet.ModelAndView;

import me.lucacirfeta.model.Element;
import me.lucacirfeta.service.ElementService;
import me.lucacirfeta.service.ServiceException;

@Controller
@RequestMapping(value = "/admin/update/{id}")
public class AdminUpdateElementController {

	@Autowired
	private ElementService elementService;

	@GetMapping
	public ModelAndView updateElement(@PathVariable(name = "id") Long id) {
		Element element = null;

		try {
			element = elementService.findById(id);

		} catch (ServiceException e) {
			System.out.println(e.getMessage());
		}
		ModelAndView mv = new ModelAndView("updateElement");
		mv.addObject("element", element);
		
		return mv;
	}

	@PostMapping
	@RequestMapping(value = "/formUpdate")
	public String formUpdateElement(@ModelAttribute Element element) {

		try {
			this.elementService.update(element);
		} catch (ServiceException e) {
			System.out.println(e.getMessage());
		}

		return "redirect:/admin/elements";
	}

}

application.properties

# ===============================
# = DATA SOURCE
# ===============================

# Set here configurations for the database connection

# Connection url for the database "netgloo_blog"
spring.datasource.url= jdbc:****

# Username and password
spring.datasource.username= ****
spring.datasource.password= ****

spring.datasource.driver-class-name= com.mysql.cj.jdbc.Driver

# Keep the connection alive if idle for a long time (needed in production)
spring.datasource.testWhileIdle = true
spring.datasource.validationQuery = SELECT 1

# ===============================
# = JPA / HIBERNATE
# ===============================

# Show or not log for each sql query
spring.jpa.show-sql = false
spring.jpa.hibernate.format_sql= true

# Hibernate ddl auto (create, create-drop, update): with "update" the database
# schema will be automatically updated accordingly to java entities found in
# the project
spring.jpa.hibernate.ddl-auto = update

# Naming strategy
spring.jpa.hibernate.naming-strategy = org.hibernate.cfg.ImprovedNamingStrategy

# Allows Hibernate to generate SQL optimized for a particular DBMS
spring.jpa.properties.hibernate.dialect= org.hibernate.dialect.MySQL5InnoDBDialect

# ===============================
# = OTHERS
# ===============================

entitymanager.packagesToScan= me.lucacirfeta

server.port=8081

server.session.timeout=15

spring.mvc.hiddenmethod.filter.enabled=true

logging.level.org.springframework.web=DEBUG

html page

<!DOCTYPE html>
<html lang="it" xmlns:th="http://www.thymeleaf.org">
<head>
<div th:replace="header :: header"></div>
<link th:href="@{/css/style.css}" rel="stylesheet" type="text/css" />
<title th:text="Aggiorna + ' ' + Elemento + ' ' + ID + ' ' + ${element.id}"></title>
</head>
<body>
<div th:insert="navbar :: navbar"></div>
<h2 style="text-align: center;">Aggiorna Elemento</h2>
<!--  Start of main body of record -->
<div class="container-fluid" style="width: 60%;">
<!--  Start of descriptive data -->
<div class="row-fluid">
<div>
<form th:action="@{'/admin/update/' + ${element.id} + '/formUpdate'}" th:object="${element}"
th:method="post">
<!-- Header of section -->
<hr>
<p><strong>ID Univoco:</strong> <span th:text="${id}"></span></p>
<div class="form-group">
<h1 class="lead">Tipo</h1>
<input th:field="*{elType.elementType}" type="text" class="form-control" />
</div>
<!-- The description of the object -->
<div>
<div class="form-group">
<h4 class="lead">Descrizione</h4>
<textarea rows="5" th:field="*{description}" class="form-control"></textarea>
</div>
</div>
<p>
</p>
<!-- Others details -->
<div class="form-group">
<h4 class="lead">Altri dettagli</h4>
<textarea th:field="*{otherDetails}" class="form-control"></textarea>
</div>
<div class="container" style="display: flex; justify-content: space-between;">
<div class="row">
<div class="col">
<h4 class="lead">Cronologia</h4>
<p>
Periodo dal: <input th:field="${element.Period.periodFrom}" class="form-control"
type="text" />
Periodo al: <input th:field="${element.Period.periodTo}" class="form-control"
type="text" />
Data dal: Circa AD <input th:field="${element.elDate.dateFrom}" class="form-control"
type="text" />
Data al: Circa AD <input th:field="${element.elDate.dateTo}" class="form-control"
type="text" />
</p>
</div>
</div>
<div class="row">
<div class="col">
<h4 class="lead">Dimensioni e Peso</h4>
<p>
Lunghezza: mm <input th:field="${element.elDimension.elLength}" class="form-control"
type="text" />
Peso: g. <input th:field="${element.elDimension.elWeight}" class="form-control"
type="text" />
Spessore: g. <input th:field="${element.elDimension.elThickness}"
class="form-control" type="text" />
Diametro: mm <input th:field="${element.elDimension.elDiameter}"
class="form-control" type="text" />
</p>
</div>
</div>
<div class="row">
<div class="col">
<h4 class="lead">Materiale</h4>
<p>
Materiale: <input th:field="${element.elType.material}" class="form-control"
type="text" />
</p>
</div>
</div>
</div>
<div class="container" style="display: flex; justify-content: space-between;">
<div class="row">
<div class="col">
<h4 class="lead">Validazione</h4>
<p>
<input th:field="${element.validation}" class="form-control" type="text" />
</p>
</div>
</div>
<div class="row">
<div class="col">
<h4 class="lead">User</h4>
<p>
<input th:field="${element.user.id}" class="form-control" type="text" readonly />
<input th:field="${element.user.username}" class="form-control" type="text"
readonly />
<input th:field="${element.user.firstName}" class="form-control" type="text"
readonly />
<input th:field="${element.user.lastName}" class="form-control" type="text"
readonly />
<input th:field="${element.user.password}" class="form-control" type="text"
readonly />
</p>
</div>
</div>
</div>
<div class="form-group">
<button class="btn btn-primary" type="submit">Aggiorna</button>
</div>
</form>
<!-- End of descriptive data -->
</div>
</div>
</div>
<div th:insert="scripts :: scripts"></div>
</body>
</html>

For such situations it is best to create a simple DTO class like ElementDTO

public class ElementDTO {
//all the fields that you need, getters, setters and default constructor
}

And then pass it to the update form by adding the following method in you controller

@ModelAttribute("element")
public ElementDTO elementDTO(){
return new ElementDTO();
}

This approach is better for several reasons:

1. You don't need to load an object from your database in advance everytime the form is loaded
2. You only need to define fields that you want to update and don't have to worry about hibernates rules on non-null fields.

Then in the controlle method expecting the object you just change the parameter to (ElementDTO elementDTO) and copy all updated fields to the actial Element object that you then save in the database

I found the solution.

I use an Element in the session added with @SessionAttributes and no lost the references with User.

This is the edited controller:

package me.lucacirfeta.controllers;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.SessionAttributes;
import me.lucacirfeta.model.Element;
import me.lucacirfeta.service.ElementService;
import me.lucacirfeta.service.ServiceException;
@Controller
@RequestMapping(value = "/admin/update/{id}")
@SessionAttributes(value = "element")
public class AdminUpdateElementController {
@Autowired
private ElementService elementService;
@ModelAttribute("element")
public Element updateElement(@PathVariable(name = "id") Long id) {
Element element = null;
try {
element = elementService.findById(id);
} catch (ServiceException e) {
System.out.println(e.getMessage());
}
return element;
}
@GetMapping
public String updateElement() {
return "updateElement";
}
@PostMapping
@RequestMapping(value = "/formUpdate")
public String formUpdateElement(Element element) {
try {
this.elementService.update(element);
} catch (ServiceException e) {
System.out.println(e.getMessage());
}
return "redirect:/admin/elements";
}
}