thymeleaf Failed to convert property value of type java.lang.String to required type java.util.List

Question

I am new to Spring and Spring Boot and am working through a book that is full of missing information.

I have a taco class:

public class Taco {

    ...

    @Size(min=1, message="You must choose at least 1 ingredient")
    private List<Ingredient> ingredients;

    ...
}

As you can see ingredients is of type List<Ingredient> and that is the problem, it used to be of type List<String> but that was before I started saving data in the database, now it must be List<Ingredient> which breaks the whole thing.

In the contoller I have the following (among other things, I think this is the only required code for the problem at hand but if you need more let me know):

@ModelAttribute
    public void addIngredientsToModel(Model model) {
        List<Ingredient> ingredients = new ArrayList<>();
        ingredientRepo.findAll().forEach(i -> ingredients.add(i));

        Type[] types = Ingredient.Type.values();
        for (Type type : types) {
            model.addAttribute(type.toString().toLowerCase(), filterByType(ingredients, type));
        }
    }

private List<Ingredient> filterByType(List<Ingredient> ingredients, Type type) {
        return ingredients
                    .stream()
                    .filter(x -> x.getType()
                    .equals(type))
                    .collect(Collectors.toList());
    }

And finally in my thymeleaf file I have:

<span class="text-danger" 
                th:if="${#fields.hasErrors('ingredients')}" 
                th:errors="*{ingredients}">
            </span>

Which causes the error:

thymeleaf Failed to convert property value of type java.lang.String to required type java.util.List

Once again, when private List<Ingredient> ingredients; was private List<String> ingredients; it worked, but now it must be private List<Ingredient> ingredients; because of the way it is saved in the database but it breaks at this point, how to fix it?

Answer 1

An optimisation of Ian's answer above:

Fetch the ingredients in the constructor of the converter.

package com.joeseff.xlabs.chtp01_1.tacos.converter;

import com.joeseff.xlabs.chtp01_1.tacos.model.Ingredient;
import com.joeseff.xlabs.chtp01_1.tacos.respository.jdbc.IngredientRepository;
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;

/**
 * @author - JoeSeff
 * @created - 23/09/2020 13:41
 */
@Component
public class IngredientConverter implements Converter<String, Ingredient> {
    private final IngredientRepository ingredientRepo;
    private final List<Ingredient> ingredients = new ArrayList<>();
    
    @Autowired
    public IngredientConverter(IngredientRepository ingredientRepo) {
        this.ingredientRepo = ingredientRepo;
    
        ingredientRepo.findAll().forEach(ingredients::add);
    }
    
    @Override
    public Ingredient convert(String ingredientId) {
        return ingredients
                .stream().filter( i -> i.getId().equals(ingredientId))
                .findFirst()
                .orElseThrow(() -> new IllegalArgumentException("Ingredient with ID '" + ingredientId + "' not found!"));
    }
}

Answer 2

I met the same problem, what we need here is a converter.

package tacos.web;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.core.convert.converter.Converter;
import org.springframework.stereotype.Component;

import tacos.Ingredient;
import tacos.data.IngredientRepository;

@Component
public class IngredientByIdConverter implements Converter<String, Ingredient> {

private IngredientRepository ingredientRepo;

@Autowired
public IngredientByIdConverter(IngredientRepository ingredientRepo) {
    this.ingredientRepo = ingredientRepo;
}

@Override
public Ingredient convert(String id) {
    return ingredientRepo.findOne(id);
}

}