One To Many

Unlike JPA, Jimmer does not support unidirectional one-to-many associations. One-to-many associations can only exist as mirrors of many-to-one associations. That is, one-to-many associations necessarily imply bidirectional associations.

In the following code:

• Left: Book.store discussed in Many To One

• Right: BookStore.books to be discussed in this article, which is the mirror of Book.store

Java

Book.java

@Entity
public interface Book {

    @ManyToOne
    @JoinColumn(name = "STORE_ID")
    BookStore store();

    ...Omit other code...
}

Kotlin

Book.kt

@Entity
interface Book {

    @ManyToOne
    @JoinColumn(name = "STORE_ID")
    val store: BookStore

    ...Omit other code...
}

Java

BookStore.java

@Entity
public interface BookStore {

    // `mappedBy` indicates `BookStore.books`
    // is the mirror of `Book.store`
    @OneToMany(mappedBy = "store")
    @Nullable
    List<Book> books();

    ...Omit other code...
}

Kotlin

BookStore.kt

@Entity
interface BookStore {

    // `mappedBy` indicates `BookStore.books`
    // is the mirror of `Book.store` 
    @OneToMany(mappedBy = "store")
    val books: List<Book>?

    ...Omit other code...
}

caution

• @OneToMany associations are merely mirrors of @ManyToOne associations. @JoinColumn and @JoinTable must not be used.

• The @OneToMany association property must be non-null. If the parent object is queried and its one-to-many association property needs to be fetched, then for the parent object that has no corresponding child objects, the value of this property is a collection of length 0 rather than null.