Bidirectional OneToMany-ManyToOne Relationship referencing unsaved transient instance (Spring MVC - Thymeleaf) - spring-boot

Question

new here. I'm new to Spring and Thymeleaf, I'm trying to learn by following a video and I don't know why I get the following exception (org.hibernate.TransientPropertyValueException: object references an unsaved transient instance - save the transient instance before flushing : org.launchcode.codingevents.models.Event.eventCategory -> org.launchcode.codingevents.models.EventCategory) when I try to creat an Event giving it an EventCategory in the Thymeleaf form. I tried cascading from one side, then from the other and then from both, but it didn't work.
I'll be immensely grateful with whoever helps me out.
Here's my code.
#MappedSuperclass
public abstract class AbstractEntity {
#Id
#GeneratedValue
private int id;
public int getId() {
return id;
}
#Override
public int hashCode() {
return Objects.hash(id);
}
#Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (obj == null || getClass() != obj.getClass()) {
return false;
}
AbstractEntity entity = (AbstractEntity) obj;
return this.id == entity.id;
}
#Entity
public class Event extends AbstractEntity {
#NotBlank(message = "Name is required")
#Size(min = 3, max = 50, message = "Name must be between 3 and 50 characters")
private String name;
#Size(max = 500, message = "Description too long!")
private String description;
#NotBlank(message = "Email is required")
#Email(message = "Invalid email. Try again")
private String contactEmail;
#ManyToOne
#NotNull(message = "Category is required")
private EventCategory eventCategory;
public Event() {
}
public Event(String name, String description, String contactEmail, EventCategory eventCategory) {
this.name = name;
this.description = description;
this.contactEmail = contactEmail;
this.eventCategory = eventCategory;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getDescription() {
return description;
}
public void setDescription(String description) {
this.description = description;
}
public String getContactEmail() {
return contactEmail;
}
public void setContactEmail(String contactEmail) {
this.contactEmail = contactEmail;
}
public EventCategory getEventCategory() {
return eventCategory;
}
public void setEventCategory(EventCategory eventCategory) {
this.eventCategory = eventCategory;
}
#Override
public String toString() {
return name;
}
#Entity
public class EventCategory extends AbstractEntity implements Serializable {
#Size(min = 3, message = "Name must be at least 3 characters long")
private String name;
#OneToMany(mappedBy = "eventCategory")
private final List<Event> events = new ArrayList<>();
public EventCategory() {
}
public EventCategory(#Size(min = 3, message = "Name must be at least 3 characters long") String name) {
this.name = name;
}
public List<Event> getEvents() {
return events;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
#Override
public String toString() {
return name;
#Controller
#RequestMapping("events")
public class EventController {
#Autowired
private EventRepository eventRepository;
#Autowired
private EventCategoryRepository eventCategoryRepository;
#GetMapping
public String displayAllEvents(#RequestParam(required = false) Integer categoryId, Model model) {
if (categoryId == null) {
model.addAttribute("title", "All Events");
model.addAttribute("events", eventRepository.findAll());
} else {
Optional<EventCategory> result = eventCategoryRepository.findById(categoryId);
if (!result.isPresent()) {
model.addAttribute("title", "Invalid Category Id: " + categoryId);
} else {
EventCategory category = result.get();
model.addAttribute("title", "Events in Category: " + category.getName());
model.addAttribute("events", category.getEvents());
}
}
return "events/index";
}
// Lives at /events/create
#GetMapping("create")
public String displayCreateEventForm(Model model) {
model.addAttribute("title", "Create Event");
model.addAttribute(new Event());
model.addAttribute("categories", eventCategoryRepository.findAll());
return "events/create";
}
// lives at /events/create
#PostMapping("create")
public String processCreateEventForm(#Valid #ModelAttribute("newEvent") Event newEvent, Errors errors, Model model) {
if (errors.hasErrors()) {
model.addAttribute("title", "Create Event");
return "events/create";
}
model.addAttribute("events", eventRepository.findAll());
eventRepository.save(newEvent);
return "redirect:";
}
// lives at /events/delete
#GetMapping("delete")
public String displayDeleteEventForm(Model model) {
model.addAttribute("title", "Delete Events");
model.addAttribute("events", eventRepository.findAll());
return "events/delete";
}
// lives at /events/delete
#PostMapping("delete")
public String processDeleteEventForm(#RequestParam(required = false) int[] eventIds) {
if (eventIds != null) {
for (int id : eventIds) {
eventRepository.deleteById(id);
}
}
return "redirect:";
}
}
Create Event
<nav th:replace="fragments :: navigation"></nav>
<form method="post" th:action="#{/events/create}" th:object="${event}">
<div class="form-group">
<label>Name
<input class="form-control" th:field="${event.name}">
</label>
<p class="error" th:errors="${event.name}"></p>
</div>
<div class="form-group">
<label>Description
<input class="form-control" th:field="${event.description}">
</label>
<p class="error" th:errors="${event.description}"></p>
</div>
<div class="form-group">
<label>Contact Email
<input class="form-control" th:field="${event.contactEmail}">
</label>
<p class="error" th:errors="${event.contactEmail}"></p>
</div>
<div class="form-group">
<label>Category
<select th:field="${event.eventCategory}">
<option th:each="eventCategory : ${categories}" th:value="${eventCategory.id}"
th:text="${eventCategory.name}">
</option>
</select>
<p class="error" th:errors="${event.eventCategory}"></p>
</label>
</div>
<div th:replace="fragments :: create-button"></div>
</form>

Answer 1

As per your code you are only trying to save Event entity and ignoring EventCategory.
You need to set Event to EventCategory as well as EventCategory to Event and make the cascade save.
First add cascade property in Event entity as below.
#ManyToOne(cascade = CascadeType.ALL)
#NotNull(message = "Category is required")
private EventCategory eventCategory;
Then in the Controller make the following changes.
#PostMapping("create")
public String processCreateEventForm(#Valid #ModelAttribute("newEvent") Event newEvent, Errors errors, Model model) {
if (errors.hasErrors()) {
model.addAttribute("title", "Create Event");
return "events/create";
}
model.addAttribute("events", eventRepository.findAll());
EventCategory eventCategory = newEvent.getEventCategory();
eventCategory.setEvent(newEvent);
eventRepository.save(newEvent);
return "redirect:";
}