I have a Springboot project. Its also using Hibernate and Thymeleaf - and I am experiencing different behavior in my form depending whether I use a Set or List between my two domain objects Episode and Person and the relationship between them is held in a entity (EpisodePerson) as there is an additional field (EpisodeRole) I record on the join.

Person

@Entity
public class Person {

private String name;
etc...

Episode

@Entity
public class Episode {

@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
@OneToMany(mappedBy = "episode", fetch = FetchType.EAGER, cascade = CascadeType.ALL)
private List<EpisodePerson> persons = new ArrayList<EpisodePerson>();

EpisodePerson

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

@ManyToOne
private Person person;

@ManyToOne
private Episode episode;

public enum EpisodeRole {
Goodie, Baddie, Witness,
}

@Column
@Enumerated(EnumType.STRING)
private EpisodeRole episodeRole;

My Episode form in html has a group of fields for capturing details about each person. There is always a minimum of two people on the form, so person[0] and person[1] are hard coded.

Any more people are added via a button which uses jquery to clone the person fieldset. I have been using the same form to display existing Episodes as I do to add a new Episode.

A typical HTML line looks like:

<input type="text" class="form-control person" data-property="firstname" placeholder="Firstname"
name="firstname" th:field="*{persons[1].person.firstname}"/>

Which brings me to the question- why is it that if my domain objects are setup to use Set then I get array out of bounds when loading the page for a new Episode?

But if using List there is no error?

It maybe that there is a better approach to the whole way people are displayed on the form.

Thanks

That's a question for the Thymeleaf forum and how they manage to implement the persons[1] syntax. For a Set, what would that mean since there is no index-based ordering?

Anyway, changing the type of a persisted collections from List to Set just to please Thymeleaf shouldn't be the right approach when designing your system. You also have to take into consideration whether you need a certain ordering.

Without application or database-level ordering, there is no such guarantee. So, why would persons[0] or persons[1] mean when you have 3 elements. If you didn't specify a certain order, you cannot get consistent results.

So the episode always has a minimum of two people, often more (average about 4).

I thought I could send episode to the view with two empty people to satisfy the first two - but this solution I do not think is very good.

What do you suggest?

Al

Nope, it's not. Why don't you use a for-loop in the UI to display the people collection? You don't need to assume any particular size. Just display dinamically whatever is coming from the DB.

There is a th:each - but persons 1 and 2 need to be in a certain part of the form, and 3 on wards in another part.

I could combine th:if (condition past index 3) for one part of the form and corresponding part for the first two.

Its working now with lists, which on the face of it should be better sinced they are indexed and kept in order? (I dont want the order of the persons in the list to change)

I'd do it as you suggested. You can use the iteration index so that the first and the second entries are displayed separately from the rest.

Ok, so when a new episode is sent to the view with no persons attached, I would need a 'add person' button. Or, as I said because it has at least two people I can initalize person 0 and person 1?

That depends on your app logic. If there is no person attached, then the for-each block will not be executed and you won't get any exception. If there must be 2 entries, then the user must fill in the data for those persons, right? So, you need to allow the user to enter that data.

However, this has nothing to do with Hibernate, so you should ask this question on the Thymeleaf forum to see how you can do this according to your application requirements.

There is a lot of talk about why NOT to use Set in MVC :

https://stackoverflow.com/questions/170 ... lating-set

And perhaps that backing object is the answer:

https://stackoverflow.com/questions/952 ... l-problems

That way is easier to encapsulate the logic and make sure you always have at least two persons for Episode.