How to create a many to many relationship with extra columns in jhipster?

Question

The jhipster doesn't support create many to many relationships with extra fields.

What is the best way to create many to many association with extra columns in jhispter? Should i create a two one-to-many relationship with extra fields?

Answer 1

Using JHipster Domain Language (JDL), a @ManytoMany holding extra properties (columns) can be easily achieved using an association entity and two ManyToOne relationships. See below:

entity Foo{
    ...
}

entity Bar{
    ...
}

entity FooBarAssociation{
    extraProperty1 String
    extraProperty2 String
    ...
}

relationship ManyToOne {
  FooBarAssociation{foo} to Foo{bars}
  FooBarAssociation{bar} to Bar{foos}
}

Answer 2

You will have to do it manually. this post describes how: https://hellokoding.com/jpa-many-to-many-extra-columns-relationship-mapping-example-with-spring-boot-maven-and-mysql/

In general, as @Antares42 said, you should create an entity for the Many-To-Many table like so:

first entity:

@Entity
public class Book{
private int id;
private String name;
private Set<BookPublisher> bookPublishers;

public Book() {
}

public Book(String name) {
    this.name = name;
    bookPublishers = new HashSet<>();
}

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
public int getId() {
    return id;
}

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

public String getName() {
    return name;
}

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

@OneToMany(mappedBy = "book", cascade = CascadeType.ALL, orphanRemoval = true)
public Set<BookPublisher>   getBookPublishers() {
    return bookPublishers;
}

public void setBookPublishers(Set<BookPublisher> bookPublishers) {
    this.bookPublishers = bookPublishers;
}
}

secound entity:

@Entity
public class Publisher {
private int id;
private String name;
private Set<BookPublisher> bookPublishers;

public Publisher(){

}

public Publisher(String name){
    this.name = name;
}

@Id
@GeneratedValue(strategy = GenerationType.AUTO)
public int getId() {
    return id;
}

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

public String getName() {
    return name;
}

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

@OneToMany(mappedBy = "publisher")
public Set<BookPublisher> getBookPublishers() {
    return bookPublishers;
}

public void setBookPublishers(Set<BookPublisher> bookPublishers) {
    this.bookPublishers = bookPublishers;
}
}

Join table entity:

@Entity
@Table(name = "book_publisher")
public class BookPublisher implements Serializable{
private Book book;
private Publisher publisher;
private Date publishedDate;

@Id
@ManyToOne
@JoinColumn(name = "book_id")
public Book getBook() {
    return book;
}

public void setBook(Book book) {
    this.book = book;
}

@Id
@ManyToOne
@JoinColumn(name = "publisher_id")
public Publisher getPublisher() {
    return publisher;
}

public void setPublisher(Publisher publisher) {
    this.publisher = publisher;
}

@Column(name = "published_date")
public Date getPublishedDate() {
    return publishedDate;
}

public void setPublishedDate(Date publishedDate) {
    this.publishedDate = publishedDate;
}
}

This entity describes the relationship between Book and Publisher and the extra field is published_date

Answer 3

Let's say you have entities like Movie, Rater and needs a join table Ratings. You can write a JDL script like the following:

entity Movie { title String}
entity Rater { name String}
entity Rating { value Integer} //the extra field

relationship ManyToMany {
   Rating{rater(name)} to Rater,
   Rating{movie(title)} to Movie
}

save it in file.jdl in the project folder, open cmd type

jhipster import-jdl file.jdl

and you have everything