spring boot String converter to Object fails

Question

I am building an app where I need to display list of categories to select from drop-down menu. What I do is:

 @ModelAttribute("categories")
public List<Category> getAllCategories(){
    return categoryRepository.findAll();
}

I have entities: Product, Category - they are ManyToOne (products->category). When I select any of the drop-down options I get :

 Failed to convert property value of type java.lang.String to required type com.slodkacysia.bakeryshop.entity.Category for property category; nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [@javax.persistence.ManyToOne com.slodkacysia.bakeryshop.entity.Category] for value 2; nested exception is org.springframework.dao.InvalidDataAccessApiUsageException: Provided id of the wrong type for class com.slodkacysia.bakeryshop.entity.Category. Expected: class java.lang.Long, got class java.lang.Integer; nested exception is java.lang.IllegalArgumentException: Provided id of the wrong type for class com.slodkacysia.bakeryshop.entity.Category. Expected: class java.lang.Long, got class java.lang.Integer

I have tried with the following converter:

@Configuration
public class WebAppConfig implements WebMvcConfigurer {
    @Override
    public void addViewControllers(ViewControllerRegistry registry) {

    }
    @Override
    public void addFormatters(FormatterRegistry registry) {
        registry.addConverter(CategoryConverter());
    }
    @Bean
    public Converter CategoryConverter(){
        return new CategoryConverter();
    }
}


import com.slodkacysia.bakeryshop.entity.Category;
import com.slodkacysia.bakeryshop.repository.CategoryRepository;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.core.convert.converter.Converter;
import org.springframework.stereotype.Component;


import java.util.ArrayList;
import java.util.List;
@Component
public class CategoryConverter implements Converter<String, List<Category>> {

    @Autowired
    private CategoryRepository categoryRepository;



    @Override
    public List<Category> convert(String s) {
        List<Category> result = new ArrayList<>();
        result.add(categoryRepository.findById(Long.parseLong(s)));
        return result;
    }

}

entities:

package com.slodkacysia.bakeryshop.entity;

import com.fasterxml.jackson.annotation.JsonIgnore;

import javax.persistence.*;
import java.math.BigDecimal;
import java.util.List;

@Entity
public class Product {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;


    private String name;
    private String description;

    private String image_url;

    private BigDecimal price;

    public List<CartItem> getCartItems() {
        return cartItems;
    }

    public void setCartItems(List<CartItem> cartItems) {
        this.cartItems = cartItems;
    }
    @ManyToOne
    private Category category;

    @OneToMany(mappedBy = "product")
    private List<CartItem> cartItems;

    public Category getCategory() {
        return category;
    }

    public void setCategory(Category category) {
        this.category = category;
    }

    private Integer available_quantity;




    public Long getId() {
        return id;
    }

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

    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 getImage_url() {
        return image_url;
    }

    public void setImage_url(String image_url) {
        this.image_url = image_url;
    }

    public BigDecimal getPrice() {
        return price;
    }

    public void setPrice(BigDecimal price) {
        this.price = price;
    }

    public Integer getAvailable_quantity() {
        return available_quantity;
    }

    public void setAvailable_quantity(Integer available_quantity) {
        this.available_quantity = available_quantity;
    }
}



package com.slodkacysia.bakeryshop.entity;

import javax.persistence.*;

@Entity
public class Category {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    private String name;


    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }


    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }
    public String getFullNameCategory() {
        return name;
    }

    @Override
    public String toString() {
        return "Category{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

and view:

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%@ taglib prefix="form" uri="http://www.springframework.org/tags/form" %>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@ taglib prefix="sec" uri="http://www.springframework.org/security/tags" %>
<html>
<head>
    <title>Product administration</title>
    <style>
        .error {
            color: red;
        }
    </style>
</head>
<a>
    <%@include file="/WEB-INF/header.jsp" %></a><br>
<%--@elvariable id="product" type="pl.coderslab.entity.Product"--%>
<form:form modelAttribute="product" >
    Nazwa <form:input path="name"/>
    <form:errors path="name" cssClass="error" /><br>

    Opis <form:input path="description" />
    <form:errors path="description" cssClass="error" /><br>

    Image URL <form:input path="image_url" />
    <form:errors path="image_url" cssClass="error" /><br>

    <br/>

    Price <form:input path="price"/>
    <form:errors path="price" cssClass="error" /><br>

    <br>
    Available Quantity <form:input path="available_quantity"/>
    <form:errors path="available_quantity" cssClass="error" /><br>

    <br>
    <form:select path="category" items="${categories}" itemLabel="fullNameCategory" itemValue="id"/>
    <form:errors path="category" cssClass="error" /><br>

    <br>
    <input type="submit">
</form:form>
</body>
</html>

Question: what do I need to get rid of these errors? thank you

Answer 1

I managed to resolve the issue in an ugly way. By adding property to category object it allowed to push the contnent of the form to the controller.