Can't pass an object from one Thymeleaf view to another Thymeleaf view - spring

Question

I'm new to Spring, and I'm working on a Spring MVC + Thymeleaf application that tracks personal expenses. I have a page that shows the list of expenses, and upon clicking "Update" on any given expense, it would show a pre-populated form, on another page, with that expense's information. The problem I'm having is that I want to pass that expense object forward to this second page, but since the data was already fetched from the database in my first page, I wouldn't want to fetch it again from my Spring Data JPA repository.
I think the unusual part of my objective is that I'm trying to pass the object like this:
Controller -> Thymeleaf -> Controller -> Thymeleaf
While the controller has sent an Expenses object to the list of expenses page, I'm trying to resend that same object (or just the one Expense, even better) back to the controller, so it would populate the model in my second form page.
My Expense entity:
package com.williampoletto.expensetracker.entity;
import java.time.LocalDate;
import java.util.Set;
import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.JoinTable;
import javax.persistence.ManyToMany;
import javax.persistence.ManyToOne;
import javax.persistence.Table;
import javax.validation.constraints.NotBlank;
import javax.validation.constraints.NotNull;
import lombok.AllArgsConstructor;
import lombok.EqualsAndHashCode;
import lombok.Getter;
import lombok.NoArgsConstructor;
import lombok.Setter;
import lombok.ToString;
#NoArgsConstructor
#AllArgsConstructor
#Getter
#Setter
#EqualsAndHashCode(of= "id")
#ToString(exclude="categories")
#Entity
#Table
public class Expense {
#Id
#GeneratedValue(strategy = GenerationType.IDENTITY )
private int id;
#NotBlank
private String description;
#NotNull
private double value;
#NotNull
private LocalDate date;
private String note;
#ManyToOne
private User user;
#ManyToMany(cascade=CascadeType.PERSIST)
#JoinTable(
name="expense_category",
joinColumns=#JoinColumn(name="expense_id"),
inverseJoinColumns=#JoinColumn(name="category_id"))
private Set<Category> categories;
public void addCategory(Category category) {
categories.add(category);
}
public void removeCategory(Category category) {
categories.remove(category);
}
public Expense(int id, #NotBlank String description, #NotNull double value, #NotNull LocalDate date,
String note, Set<Category> categories) {
super();
this.id = id;
this.description = description;
this.value = value;
this.date = date;
this.note = note;
this.categories = categories;
}
}
My thymeleaf table:
<table class="table table-bordered table-striped">
<thead class="thead-dark">
<tr>
<th>Description</th>
<th>Value</th>
<th>Category</th>
<th>Note</th>
<th>Date</th>
<th>Action</th>
</tr>
</thead>
<tbody>
<tr th:each="expense : ${expenses}">
<td th:text="${expense.description}"/>
<td th:text="${expense.value}"/>
<td>
<div th:each="category : ${expense.categories}">
<p th:text ="${category.name}"></p>
</div>
</td>
<td th:text="${expense.note}"/>
<td th:text="${expense.date}"/>
<td>
<form th:action="#{/expenses/showUpdateForm}" th:object="${expense}" method="POST">
<input type="text" name="expense" th:value="${expense.id}">
<input type="text" name="expenseDescription" th:value="${expense.description}">
<input type="text" name="expenseValue" th:value="${expense.value}">
<input type="text" name="expenseDate" th:value="${expense.date}">
<input type="text" name="expenseNote" th:value="${expense.note}">
<input type="text" name="expenseCategories" th:value="${expense.categories}">
<button th:if="${expense.user != null}" type="submit" class="btn btn-info btn-sm">Update</button>
</form>
<a th:href="#{/expenses/delete(expenseId=${expense.id})}"
class="btn btn-danger btn-sm"
onclick="if (!(confirm('Are you sure you want to delete this expense?'))) return false">Delete</a>
</td>
</tbody>
</table>
My /list and /showUpdateForm code for the controller:
#GetMapping("/list")
public String list(#AuthenticationPrincipal UserDetailsImpl userDetails, Model model) {
Set<Expense> expenses = expenseService.findAll(userDetails.getUserId());
model.addAttribute("expenses", expenses);
return "expenses";
}
#PostMapping("/showUpdateForm")
public String showFormForUpdate(#RequestParam("expenseId") int expenseId,
#RequestParam("expenseDescription") String expenseDescription,
#RequestParam("expenseValue") double expenseValue,
#RequestParam("expenseDate") String expenseDate,
#RequestParam("expenseNote") String expenseNote,
#RequestParam("expenseCategories") Set<Category> expenseCategories,
Model model) {
Expense expense = new Expense
(expenseId, expenseDescription, expenseValue, expenseDate, expenseNote, expenseCategories);
model.addAttribute("expense", expense);
return "/expense-form";
}
My final form page:
<form th:action="#{/expenses/save}" th:object="${expense}" method="POST">
<input type="hidden" th:field="*{id}">
<label>Description</label>
<input type="text" th:field="*{description}" class="form-control mb-4 col-4" placeholder="Description">
<label>Value</label>
<input type="text" th:field="*{value}" class="form-control mb-4 col-4" placeholder="Value">
<label>Date</label>
<input type="text" th:field="*{date}" class="form-control mb-4 col-4" placeholder="Date">
<label>Note</label>
<input type="text" th:field="*{note}" class="form-control mb-4 col-4" placeholder="Note">
<label>Categories</label>
<input type="hidden"
th:each="category : *{categories}"
th:value="${category.id}"
th:field="*{categories}"
th:text="${category.name}"/>
<button type="submit" class="btn btn-success col-2 mb-2">Save</button>
</form>
What I have tried:
Instead of sending an object, sending individual values as you can see in my thymeleaf table. The problem with this is that my Expense entity has a LocalDate, User (another entity) and Set attributes, so I had trouble converting these to an Expense object on the controller side. Ideally I would want to write something like this to simply pass on the object, but this example sends a toString:
<form th:action="#{/expenses/showUpdateForm}" th:object="${expense}" method="POST">
<input type="text" name="expense" th:value="${expense}">
</form>
Tried to use an argument to /showUpdateForm to hopefully get the expenses object from the model:
#RequestAttribute("expenses") Set<Expense> expenses
Tried to retrieve the object like this in /showUpdateForm, with the same intent as 2:
Set<Expense> expenses = (Set<Expense>) model.getAttribute("expenses");
Tried to use RedirectAttributes in the controller, which I saw can be useful for passing objects between controllers, but maybe not in my case:
#GetMapping("/showUpdateForm")
public String showFormForUpdate(Model model, RedirectAttributes attributes) {
attributes.addAttribute("expenses");
return "/expense-form";
}
Anyway, I have no idea how to achieve this, I would appreciate any light on the subject! I know that I can easily fix this by simply sending an id from view to controller, then I could perform a repository search of the object with that ID, but that would be cumbersome to the database since the data already exists and was fetched in the previous page.