How to pass value from drop-down list - spring-mvc

Question

I'm working with the Spring MVC + Thymeleaf project, and I have a problem with passing the field value to object.
There are the malt and country entities. In malt form, there is a drop-down list, that are populated from DB - only country names - nothig fancy. I'm able to populate the list, but when I'm clicking "submit" button, there are some errors. Code below (only relevant parts):
Malt entity:
#Setter
#Getter
#NoArgsConstructor
#Entity
#ToString
#Table(name="malt")
public class Malt extends BaseEntity {
#Column(name="malt_name")
private String maltName;
#ManyToOne(fetch=FetchType.EAGER,
cascade= {CascadeType.PERSIST, CascadeType.MERGE,
CascadeType.DETACH, CascadeType.REFRESH})
#JoinColumn(name="producer_id")
private Producer producer;
#Column(name="malt_filling")
private int maltFilling;
#Column(name="malt_ebc")
private int maltEbc;
#Column(name="malt_usage")
private String maltUsage;
#ManyToOne(fetch=FetchType.EAGER,
cascade= {CascadeType.PERSIST, CascadeType.MERGE,
CascadeType.DETACH, CascadeType.REFRESH})
#JoinColumn(name="country_id")
private Country country;
#ManyToMany(mappedBy="malts")
private Set<Batch> batches;
Malt controller:
#Controller
#RequestMapping("/malt")
public class MaltController {
#ModelAttribute("countries")
public Collection<Country> populateCountries() {
return countryService.findAll();
}
#RequestMapping("{id}/update")
public String updateMalt(#PathVariable String id, Model model) {
model.addAttribute("malt", maltService.findById(Long.valueOf(id)));
return "malt-form";
}
#PostMapping
public String saveOrUpdate(#ModelAttribute Malt malt) {
Malt savedMalt = maltService.save(malt);
return "redirect:/malt/" + savedMalt.getId() + "/malt-show";
}
Malt form:
<div class="form-field-input">
<select class="form-control" th:field="*{id}">
<option value="0">Select country</option>
<option
th:each="country : ${countries}"
th:value="${country.id}"
th:text="${country?.countryName}">
</option>
</select>
</div>
<div class="form-field-submit">
<button class="submit-button" type="submit">Submit</button>
</div>
Malt show template:
<div class="wrapper">
<div class="main">
<div class="page-title">
<p th:text="${malt.maltName}">Malt name</p>
</div>
<div class="show">
<div class="form-row">
<div class="form-field-name">
<label>Producer:</label>
</div>
<div class="form-field-input">
<p th:text="${malt.producer.producerName}">Producer name</p>
</div>
</div>
<div class="form-row">
<div class="form-field-name">
<label>Country:</label>
</div>
<div class="form-field-input">
<p th:text="${malt.country.countryName}">Country</p>
</div>
</div>
<div class="form-row">
<div class="form-field-name">
<label>Malt filling:</label>
</div>
<div class="form-field-input">
<p th:text="${malt.maltFilling}">Malt filling</p>
</div>
</div>
<div class="form-row">
<div class="form-field-name">
<label>Malt usage:</label>
</div>
<div class="form-field-input">
<p th:text="${malt.maltUsage}">Malt usage</p>
</div>
</div>
<div class="form-row">
<div class="form-field-name">
<label>Malt EBC:</label>
</div>
<div class="form-field-input">
<p th:text="${malt.maltEbc}">Malt EBC</p>
</div>
</div>
</div>
</div>
</div>
End error that I'm getting:
An error happened during template parsing (template: "class path resource [templates/malt-show.html]")
org.thymeleaf.exceptions.TemplateInputException: An error happened during template parsing (template: "class path resource [templates/malt-show.html]")
.
.
.
Caused by: org.attoparser.ParseException: Exception evaluating SpringEL expression: "malt.country.countryName" (template: "malt-show" - line 44, col 11)
at org.attoparser.MarkupParser.parseDocument(MarkupParser.java:393)
at org.attoparser.MarkupParser.parse(MarkupParser.java:257)
at org.thymeleaf.templateparser.markup.AbstractMarkupTemplateParser.parse(AbstractMarkupTemplateParser.java:230)
... 52 more
Caused by: org.thymeleaf.exceptions.TemplateProcessingException: Exception evaluating SpringEL expression: "malt.country.countryName" (template: "malt-show" - line 44, col 11)
at org.thymeleaf.spring5.expression.SPELVariableExpressionEvaluator.evaluate(SPELVariableExpressionEvaluator.java:290)
.
.
.
Caused by: org.springframework.expression.spel.SpelEvaluationException: EL1007E: Property or field 'countryName' cannot be found on null
Link to repo: https://github.com/fangirsan/maruszka-new/tree/malt-form-problem
I've tried a lot of different approaches, but with no result.

Answer 1

As the exception said, problem is ${malt.country.countryName} inside your malt-show form. In the last line of exception stacktrace, I see Property or field 'countryName' cannot be found on null. This means that you are trying to get a property of related model that is null. Probably, the column country_id in your malt table is null. In the other words, country_id not saved with the other fields. With these assumptions, you will find the problem in the malt-form form where you are saving malts. I inspected this form, probably the problem is <select class="form-control" th:field="*{id}">. I think this must be changed to <select class="form-control" th:field="*{country.id}">.
Important Note:
Some of related models can be null, for example assume country in your malt model can be null(it depends on application business logic). In case of nullable relations, accessing the model relation fields in ${model.relation.field} pattern may produce above error. So you should use null checking inside your thymeleaf template in those cases.